Regulatory Matters	12 Months Ended
Dec. 31, 2013
Regulatory Matters [Line Items]
Regulatory Matters	REGULATORY MATTERSRegulatory Assets - At December 31, regulatory assets were comprised of the following items (in millions): Alliant Energy IPL WPL 2013 2012 2013 2012 2013 2012Tax-related$829.7 $770.7 $798.6 $746.2 $31.1 $24.5Pension and other postretirement benefits costs355.3 549.2 174.2 279.3 181.1 269.9AROs65.7 62.4 36.7 38.6 29.0 23.8Emission allowances30.0 30.0 30.0 30.0 — —Environmental-related costs25.0 34.9 21.0 30.3 4.0 4.6Derivatives21.1 40.2 5.9 16.3 15.2 23.9Debt redemption costs17.9 19.8 12.2 13.6 5.7 6.2IPL’s electric transmission service costs8.3 16.6 8.3 16.6 — —Other60.2 88.6 26.6 47.0 33.6 41.6 $1,413.2 $1,612.4 $1,113.5 $1,217.9 $299.7 $394.5A portion of the regulatory assets in the above table are not earning a return. These regulatory assets are expected to be recovered from customers in future rates; however, the respective carrying costs of these assets are not expected to be recovered from customers in future rates. At December 31, 2013, IPL and WPL had $39 million and $13 million, respectively, of regulatory assets representing past expenditures that were not earning a return. IPL’s regulatory assets that were not earning a return consisted primarily of debt redemption costs and electric transmission service costs. WPL’s regulatory assets that were not earning a return consisted primarily of amounts related to wholesale customer rate recovery, which is discussed in Note 1(f). The other regulatory assets reported in the above table either earn a return or the cash has not yet been expended, in which case the assets are offset by liabilities that also do not incur a carrying cost.Tax-related - IPL and WPL record regulatory assets for certain temporary differences (primarily related to utility property, plant and equipment at IPL) that result in a decrease in current rates charged to customers and an increase in future rates charged to customers based on the timing of income tax expense that is used to determine such rates. These temporary differences include the impacts of qualifying deductions for repairs expenditures and allocation of mixed service costs, and Iowa accelerated tax depreciation, which all contribute to lower current income tax expense during the first part of an asset’s useful life and higher current tax expense during the last part of an asset’s useful life. These regulatory assets will be recovered from customers in the future when these temporary differences reverse resulting in additional current income tax expense used to determine customers’ rates. During 2013, Alliant Energy’s and IPL’s “Tax-related” regulatory assets in the above table increased primarily due to qualifying repairs expenditure deductions at IPL.Pension and other postretirement benefits costs - The IUB and the PSCW have authorized IPL and WPL to record the retail portion of their respective previously unrecognized net actuarial gains and losses, and prior service costs and credits, as regulatory assets in lieu of AOCL on the Consolidated Balance Sheets, as these amounts are expected to be recovered in future rates. IPL and WPL also recognize the wholesale portion of their previously unrecognized net actuarial gains and losses, and prior service costs and credits, as regulatory assets on the Consolidated Balance Sheets because these costs are expected to be recovered in rates in future periods under the formula rate structure. These regulatory assets will be increased or decreased as the net actuarial gains or losses, and prior service costs or credits, are subsequently amortized and recognized as a component of net periodic benefit costs. Regulatory assets are also increased or decreased as a result of the annual defined benefit plan measurement process. During 2013, Alliant Energy’s, IPL’s and WPL’s pension and other postretirement benefits costs regulatory assets decreased due to a decrease in unrecognized net actuarial losses caused by higher discount rates and higher returns on assets compared to assumptions used in the annual defined benefit plan measurement process as of December 31, 2012.Pension and other postretirement benefits costs are included within the recoverable cost of service component of rates charged to IPL’s and WPL’s customers. The recoverable costs included in customers’ rates are based upon pension and other postretirement benefits costs determined in accordance with GAAP and are calculated using different methods for the various regulatory jurisdictions in which IPL and WPL operate. The methods for IPL’s and WPL’s primary regulatory jurisdictions are described below. The IUB authorized IPL in its most recent Iowa retail electric rate case order to recover from its retail electric customers in Iowa an allocated portion of annual costs equal to a two-year simple average of actual costs incurred during its test year (2009) and an estimate of costs for its forward-looking post-test year (2010). The PSCW authorized WPL in its most recent Wisconsin retail rate case to recover from its electric and gas retail customers in base rates an estimated allocated portion of annual costs equal to the costs expected to be incurred during the 2013 and 2014 test year period. WPL is authorized to recover from its wholesale customers an allocated portion of actual pension costs incurred each year. In accordance with FERC-approved formula rates, any over- or under-collection of these costs each year are refunded to or recovered from customers through subsequent changes to wholesale customer rates. WPL is authorized to recover from its wholesale customers an allocated portion of other postretirement benefits costs based on the amount of other postretirement benefits costs incurred in 2006. Refer to Note 12(a) for additional details regarding pension and other postretirement benefits costs.AROs - Alliant Energy, IPL and WPL believe it is probable that any differences between expenses accrued for legal AROs related to their regulated operations and expenses recovered currently in rates will be recoverable in future rates, and are deferring the differences as regulatory assets. Refer to Note 13 for additional details of AROs.Emission allowances - IPL entered into forward contracts in 2007 to purchase SO2 emission allowances with vintage years of 2014 through 2017 from various counterparties for $34 million to meet future CAIR emission reduction standards. Any SO2 emission allowances acquired under these forward contracts may be used to meet requirements under the existing Acid Rain program regulations or the more stringent CAIR emission reduction standards but are not eligible to be used for compliance requirements under CSAPR. In 2011, the EPA issued CSAPR to replace CAIR with an anticipated effective date in 2012. As a result of the issuance of CSAPR, Alliant Energy and IPL concluded in 2011 that the allowances to be acquired under these forward contracts would not be needed by IPL to comply with expected environmental regulations in the future. The value of these allowances was nominal, which was significantly below the $34 million contract price for these allowances. As a result, Alliant Energy and IPL recognized charges of $34 million for these forward contracts in 2011 with an offsetting obligation recorded in other long-term liabilities and deferred credits. Alliant Energy and IPL concluded that $30 million of the charges are probable of recovery from IPL’s customers, and therefore, were recorded to regulatory assets in 2011. The remaining $4 million of charges were determined not to be probable of recovery from IPL’s customers resulting in $2 million of charges related to electric customers recorded to “Electric production fuel and energy purchases” and $2 million of charges related to steam customers recorded to “Utility - Other operation and maintenance” in Alliant Energy’s and IPL’s Consolidated Statements of Income in 2011. In 2012, the D.C. Circuit Court vacated and remanded CSAPR for further revision to the EPA. The D.C. Circuit Court order also requires the EPA to continue administering CAIR pending the promulgation of a valid replacement for CSAPR. Despite CSAPR being vacated, the current value of these allowances continues to be nominal and significantly below the $34 million contract price for these allowances. Alliant Energy and IPL currently believe that CAIR will be replaced in the future, either by a modified CSAPR or another rule that addresses the interstate transport of air pollutants.Environmental-related costs - The IUB has permitted IPL to recover prudently incurred costs by allowing a representative level of MGP costs in the recoverable cost of service component of rates, as determined in its most recent retail gas rate case. Under the current rate-making treatment approved by the PSCW, the MGP expenditures of WPL are deferred and collected from retail gas customers over a five-year period after new rates are implemented. The MPUC allows the deferral of MGP-related costs applicable to IPL’s Minnesota sites and IPL has received approval to recover such costs in retail gas rates in Minnesota in its most recent retail gas rate case. Regulatory assets recorded by IPL and WPL reflect the probable future rate recovery of MGP expenditures. Refer to Note 16(e) for additional details of environmental-related MGP costs.Derivatives - In accordance with IPL’s and WPL’s fuel and natural gas recovery mechanisms, prudently incurred costs from derivative instruments are recovered from customers in the future after any losses are realized and gains from derivative instruments are refunded to customers in the future after any gains are realized. Based on these recovery mechanisms, the changes in the fair value of derivative liabilities/assets resulted in comparable changes to regulatory assets/liabilities on the Consolidated Balance Sheets. Refer to Note 15 for additional details of derivative assets and derivative liabilities.Debt redemption costs - For debt retired early with no subsequent re-issuance, IPL and WPL defer any debt repayment premiums and unamortized debt issuance costs and discounts as regulatory assets. These regulatory assets are amortized over the remaining original life of the debt retired early. Debt repayment premiums and other losses resulting from the refinancing of debt by IPL and WPL are deferred as regulatory assets and amortized over the life of the new debt issued.IPL’s electric transmission service costs - In 2010, IPL incurred electric transmission service costs billed by ITC under the Attachment “O” rate for ITC’s under-recovered 2008 costs. In 2010, the IUB issued an order authorizing IPL to defer the Iowa retail portion of these under-recovered costs and amortize the deferred costs over a five-year period ending December 2014. In accordance with this order, IPL is amortizing $8 million of this regulatory asset annually, with an equal and offsetting amount of amortization of IPL’s regulatory liability related to its electric transmission assets sale. The IUB determined that IPL should not include the unamortized balance of these deferred costs in electric rate base during the five-year recovery period.Other - Alliant Energy, IPL and WPL assess whether IPL’s and WPL’s regulatory assets are probable of future recovery by considering factors such as applicable regulations, recent orders by the applicable regulatory agencies, historical treatment of similar costs by the applicable regulatory agencies and regulatory environment changes. Based on these assessments, Alliant Energy, IPL and WPL believe the regulatory assets recognized as of December 31, 2013 in the above table are probable of future recovery. However, no assurance can be made that IPL and WPL will recover all of these regulatory assets in future rates. If future recovery of a regulatory asset ceases to be probable, the regulatory asset will be charged to expense in the period in which future recovery ceases to be probable.Based on the PSCW’s July 2012 order related to WPL’s 2013/2014 test period Wisconsin retail electric and gas rate case, WPL was authorized to recover previously incurred costs associated with the acquisition of a 25% ownership interest in Edgewater Unit 5 and proposed emission controls projects. As a result, Alliant Energy and WPL recorded a $5 million increase to regulatory assets, and a $5 million credit to “Utility - Other operation and maintenance” in their Consolidated Statements of Income in 2012.Based on assessments completed in 2011, Alliant Energy, IPL and WPL recognized impairment charges of $9 million, $2 million and $7 million, respectively, for regulatory assets that were no longer probable of future recovery. The regulatory asset impairment charges were recorded by Alliant Energy, IPL and WPL as reductions in regulatory assets and charges to “Utility - Other operation and maintenance” in their Consolidated Statements of Income in 2011.Based on the MPUC’s November 2011 order related to IPL’s 2009 test year Minnesota retail electric rate case, IPL was authorized to recover $2 million of previously incurred plant cancellation costs for Sutherland #4 over a 25-year period ending in 2037. As a result, Alliant Energy and IPL recorded a $2 million increase to regulatory assets, and a $2 million credit to “Utility - Other operation and maintenance” in their Consolidated Statements of Income in 2011.Regulatory Liabilities - At December 31, regulatory liabilities were comprised of the following items (in millions): Alliant Energy IPL WPL 2013 2012 2013 2012 2013 2012Cost of removal obligations$418.9 $408.7 $277.7 $268.0 $141.2 $140.7IPL’s tax benefit riders265.4 355.8 265.4 355.8 — —Energy conservation cost recovery52.7 55.1 9.3 10.0 43.4 45.1IPL’s electric transmission assets sale21.6 32.5 21.6 32.5 — —IPL’s electric transmission cost recovery14.6 — 14.6 — — —Commodity cost recovery7.5 17.7 5.5 5.2 2.0 12.5Other40.8 46.3 20.8 29.9 20.0 16.4 $821.5 $916.1 $614.9 $701.4 $206.6 $214.7Regulatory liabilities related to cost of removal obligations, to the extent expensed through depreciation rates, reduce rate base. A significant portion of the remaining regulatory liabilities are not used to reduce rate base in the revenue requirement calculations utilized in IPL’s and WPL’s respective rate proceedings.Cost of removal obligations - Alliant Energy, IPL and WPL collect in rates future removal costs for many assets that do not have associated legal AROs. Alliant Energy, IPL and WPL record a regulatory liability for the estimated amounts they have collected in rates for these future removal costs less amounts spent on removal activities.IPL’s tax benefit riders - At December 31, 2013, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities in the above table consisted of $228.9 million and $36.5 million for the electric and gas tax benefit riders, respectively. These regulatory liabilities decreased $90 million in aggregate in 2013 due to the following items:Electric tax benefit rider - In January 2011, the IUB approved an electric tax benefit rider proposed by IPL, which utilizes regulatory liabilities to credit bills of Iowa retail electric customers beginning in February 2011 to help offset the impact of rate increases on such customers. These regulatory liabilities are related to tax benefits from tax accounting method changes for repairs expenditures, allocation of mixed service costs and allocation of insurance proceeds from floods in 2008. Alliant Energy and IPL recognize an offsetting reduction to income tax expense for the after-tax amounts credited to IPL’s retail electric customers’ bills in Iowa, resulting in no impact to Alliant Energy’s and IPL’s net income from the electric tax benefit rider. In 2013, 2012 and 2011, Alliant Energy and IPL utilized $79 million, $83 million and $61 million, respectively, of electric tax benefit rider-related regulatory liabilities accumulated in prior years to credit IPL’s Iowa retail electric customers’ bills. In 2013, 2012 and 2011, the $79 million, $83 million and $61 million reductions to “Electric operating revenues” resulted in $33 million, $35 million and $25 million of credits to “Income tax expense (benefit)” as a result of the decrease in taxable income in Alliant Energy’s and IPL’s Consolidated Statements of Income in 2013, 2012 and 2011, respectively. In 2013, 2012 and 2011, additional reductions to “Income tax expense (benefit)” of $46 million, $48 million and $36 million, respectively, were also recognized in Alliant Energy’s and IPL’s Consolidated Statements of Income representing the tax benefits realized related to the electric tax benefit rider. In December 2013, the IUB issued an order authorizing $85 million of regulatory liabilities from tax benefits to be credited to IPL’s retail electric customers’ bills in Iowa during 2014 through the electric tax benefit rider.Gas tax benefit rider - In November 2012, the IUB approved a gas tax benefit rider proposed by IPL, which utilizes up to $12 million of regulatory liabilities annually to credit bills of Iowa retail gas customers beginning in January 2013 through December 2015 to help offset the impact of rate increases on such customers. These regulatory liabilities are related to tax benefits from tax accounting method changes for repairs expenditures, allocation of mixed service costs and allocation of insurance proceeds from floods in 2008. Any remaining benefit, including any portion not utilized of the agreed upon amount from January 2013 through December 2015, will be credited to Iowa’s retail gas customers’ bills in 2016. In 2013, Alliant Energy and IPL utilized $11 million of gas tax benefit rider-related regulatory liabilities to credit IPL’s Iowa retail gas customers’ bills. In 2013, the $11 million reduction to “Gas operating revenues” resulted in $4 million of credits to “Income tax expense (benefit)” as a result of the decrease in taxable income in Alliant Energy’s and IPL’s Consolidated Statements of Income in 2013. In 2013, additional reductions to “Income tax expense (benefit)” of $7 million were also recognized in Alliant Energy’s and IPL’s Consolidated Statements of Income representing the tax benefits realized related to the gas tax benefit rider.Refer to Note 11 for additional details regarding IPL’s tax benefit riders.Energy conservation cost recovery - WPL and IPL collect revenues from their customers to offset certain expenditures incurred by WPL and IPL for conservation programs, including state mandated programs and Shared Savings programs. Differences between forecasted costs used to set rates and actual costs for these programs are deferred as a regulatory asset or regulatory liability.IPL’s electric transmission assets sale - In 2007, IPL completed the sale of its electric transmission assets to ITC and recognized a gain based on the terms of the agreement. Upon closing of the sale, IPL established a regulatory liability of $89 million pursuant to conditions established by the IUB when it allowed the transaction to proceed. The regulatory liability represented the present value of IPL’s obligation to refund to its customers payments beginning in the year IPL’s customers experience an increase in rates related to the transmission charges assessed by ITC. The regulatory liability accrues interest at the monthly average U.S. Treasury rate for three-year maturities.Iowa retail portion - In 2009, the IUB issued an order authorizing IPL to use a portion of this regulatory liability to reduce Iowa retail electric customers’ rates by $12 million for the period from July 2009 through February 2010 with billing credits included in the monthly energy adjustment clause. In 2010, the IUB issued an order authorizing IPL to use a portion of this regulatory liability to offset electric transmission service costs expected to be billed to IPL by ITC in 2010 related to ITC’s 2008 transmission revenue adjustment. IPL expects to utilize $41 million of this regulatory liability over a five-year period ending December 2014 to offset the Iowa retail portion of transmission costs billed to IPL by ITC in 2010 related to ITC’s 2008 transmission revenue adjustment. As a result, IPL is amortizing $8 million of this regulatory liability annually, with an equal and offsetting amount of amortization for IPL’s regulatory asset related to electric transmission service costs.In accordance with the IUB’s 2011 order related to IPL’s 2009 test year Iowa retail electric rate case, IPL was authorized to utilize regulatory liabilities in 2011 to offset transmission service expenses related to the Iowa retail portion of 2009 under-recovered costs billed to IPL. As a result, Alliant Energy and IPL recorded a reduction of $19 million in regulatory liabilities, and a reduction of $19 million in “Electric transmission service” in their Consolidated Statements of Income in 2011. The IUB also authorized IPL to utilize $3 million of this regulatory liability in 2011 to reduce IPL’s Iowa retail electric rate base associated with the Whispering Willow - East wind project.Minnesota retail portion - In 2010, the MPUC issued an interim rate order authorizing IPL to use a portion of this regulatory liability to implement an alternative transaction adjustment through its energy adjustment clause resulting in annual credits to its Minnesota retail electric customers beginning in July 2010 to coincide with the effective date of the interim rate increase for Minnesota retail customers. The amounts of the annual credits are dependent upon the level of KWhs sold to IPL’s Minnesota retail customers. In accordance with the MPUC’s November 2011 order related to IPL’s 2009 test year Minnesota retail electric rate case, IPL was authorized to refund a higher amount of the gain realized from the sale of its electric transmission assets in 2007 to its Minnesota retail electric customers than previously estimated. As a result, Alliant Energy and IPL recorded a $5 million increase to regulatory liabilities, and a $5 million charge to “Utility - Other operation and maintenance” in their Consolidated Statements of Income in 2011 for the additional amount to be refunded.Refunds related to any remaining balance of IPL’s electric transmission assets sale regulatory liability are expected to be determined in future rate proceedings or as part of the proposed sale of Minnesota electric distribution assets.IPL’s electric transmission cost recovery - Refer to Note 1(g) for additional details of IPL’s electric transmission service cost recovery mechanism.Commodity cost recovery - Refer to Note 1(g) for additional details of IPL’s and WPL’s cost recovery mechanisms. Refer to “WPL’s Retail Fuel-related Rate Filing (2012 Test Year)” below for discussion of refunds made to WPL’s retail electric customers in 2013.Utility Rate Cases -WPL’s Wisconsin Retail Electric and Gas Rate Case (2013/2014 Test Period) - In July 2012, WPL received an order from the PSCW authorizing WPL to implement a decrease in annual base rates for WPL’s retail gas customers of $13 million effective January 1, 2013, followed by a freeze of such gas base rates through the end of 2014. The order also authorized WPL to maintain customer base rates for its retail electric customers at their current levels through the end of 2014. The order included provisions that require WPL to defer a portion of its earnings if its annual regulatory return on common equity exceeds certain levels during 2013 or 2014 and allows WPL to request a change in retail base rates during this period if its annual regulatory return on common equity falls below a certain level. As of December 31, 2013, Alliant Energy and WPL did not record any material deferred amounts for these provisions. Refer to “Regulatory Assets” above for discussion of regulatory-related credits recorded in 2012 as a result of the PSCW’s order authorizing WPL to recover previously incurred costs associated with the acquisition of a 25% interest in Edgewater Unit 5 and proposed emission controls projects.IPL’s Iowa Retail Gas Rate Case (2011 Test Year) - In May 2012, IPL filed a request with the IUB to increase annual rates for its Iowa retail gas customers based on a 2011 historical test year as adjusted for certain known and measurable changes occurring up to 12 months after the commencement of the proceeding. IPL’s request included a proposal to utilize regulatory liabilities to credit bills of Iowa retail gas customers to help mitigate the impact of the proposed final rate increase on such customers. IPL proposed to reduce customer bills utilizing a gas tax benefit rider over a three-year period by approximately $36 million in aggregate. In conjunction with the filing, IPL implemented an interim retail gas rate increase of $9 million, or approximately 3%, on an annual basis, effective June 4, 2012, without regulatory review and subject to refund pending determination of final rates from the request. In 2012, Alliant Energy and IPL recorded $5 million in gas revenues from IPL’s Iowa retail gas customers related to the interim retail gas rate increase. In November 2012, the IUB approved a settlement agreement between IPL, the OCA and the Iowa Consumers Coalition related to IPL’s request, resulting in a final increase in annual rates for IPL’s Iowa retail gas customers of $11 million, or approximately 4%, effective January 10, 2013. The parties and the IUB also agreed to IPL’s proposed gas tax benefit rider. In 2013, Alliant Energy and IPL recorded $11 million in gas revenues from IPL’s Iowa retail gas customers related to the final retail gas rate increase. Refer to “Regulatory Liabilities” above for additional details on IPL’s gas tax benefit rider.IPL’s Iowa Retail Electric Rate Case (2009 Test Year) - Electric Tax Benefit Rider - In February 2013, the IUB issued an order allowing IPL to recognize a revenue requirement adjustment of $24 million in 2013 related to certain tax benefits from tax accounting method changes. The revenue requirement adjustment is recognized through the energy adjustment clause as a reduction of the credits on IPL’s Iowa retail electric customers’ bills from the electric tax benefit rider. In 2013, Alliant Energy and IPL recognized $24 million of the revenue requirement adjustment resulting in increases to electric revenues in their Consolidated Statements of Income. In December 2013, the IUB authorized IPL to reduce the billing credits on customers’ bills by $15 million in 2014 from tax benefits for the electric tax benefit rider to recognize the revenue requirement impact of the changes in tax accounting methods.IPL’s Minnesota Retail Electric Rate Case (2009 Test Year) - In May 2010, IPL filed a request with the MPUC to increase annual rates for its Minnesota retail electric customers based on a 2009 historical test year as adjusted for certain known and measurable items at the time of the filing. The key drivers for the filing included recovery of investments in the Whispering Willow - East wind project and emission controls projects at Lansing Unit 4, and recovery of increased electric transmission service costs. In conjunction with the filing, IPL implemented an interim retail rate increase of $14 million, on an annual basis, effective July 6, 2010. In November 2011, IPL received an order from the MPUC authorizing a final annual retail electric rate increase equivalent to $11 million. The final annual retail electric rate increase of $11 million includes $8 million of higher base rates, $2 million from the temporary renewable energy rider and $1 million from the utilization of regulatory liabilities to offset higher electric transmission service costs. Refer to “Regulatory Assets” and “Regulatory Liabilities” above for discussion of changes to regulatory assets and regulatory liabilities in 2011 based on the MPUC’s decisions in this rate case. Refer to Note 3(a) for discussion of adjustments made by Alliant Energy and IPL in 2011 and 2013 to the carrying value of IPL’s Whispering Willow - East wind project, based on amounts IPL determined were probable of being disallowed for recovery from its Minnesota retail electric customers.WPL’s Retail Fuel-related Rate Filing (2014 Test Year) - In December 2013, WPL received an order from the PSCW authorizing an annual retail electric rate increase of $19 million, or approximately 2%, effective January 1, 2014 to reflect anticipated increases in retail fuel-related costs in 2014 compared to the fuel-related cost estimates used to determine rates for 2013. WPL’s 2014 fuel-related costs will be subject to deferral if they fall outside an annual bandwidth of plus or minus 2% of the approved annual forecasted fuel-related costs.WPL’s Retail Fuel-related Rate Filing (2013 Test Year) - In December 2012, WPL received an order from the PSCW authorizing an annual retail electric rate decrease of $29 million, or approximately 3%, effective January 1, 2013 to reflect anticipated decreases in retail fuel-related costs in 2013 compared to the fuel-related cost estimates used to determine rates for 2012. WPL’s 2013 fuel-related costs were subject to deferral if they fell outside an annual bandwidth of plus or minus 2% of the approved annual forecasted fuel-related costs. Retail fuel-related costs incurred by WPL for 2013 did not fall outside of the fuel monitoring range.WPL’s Retail Fuel-related Rate Filing (2012 Test Year) - In December 2011, WPL received an order from the PSCW authorizing an annual retail electric rate increase of $4 million, effective January 1, 2012 to reflect anticipated increases in retail fuel-related costs in 2012 compared to the fuel-related cost estimates used to determine rates for 2011. The 2012 fuel-related costs were subject to an annual bandwidth of plus or minus 2%. Retail fuel-related costs incurred by WPL in 2012 were lower than retail fuel-related costs used to determine rates for such period resulting in an over-collection of fuel-related costs for 2012 of approximately $17 million (including $11 million outside the approved range for 2012 recorded in “Regulatory liabilities” on Alliant Energy’s and WPL’s Consolidated Balance Sheets as of December 31, 2012). In August 2013, WPL received an order from the PSCW to refund $12 million, including interest, to its retail electric customers for these over-collections, which WPL completed in September 2013.Refer to Note 1(g) for further discussion of WPL’s fuel cost recovery mechanism.
IPL [Member]
Regulatory Matters [Line Items]
Regulatory Matters	REGULATORY MATTERSRegulatory Assets - At December 31, regulatory assets were comprised of the following items (in millions): Alliant Energy IPL WPL 2013 2012 2013 2012 2013 2012Tax-related$829.7 $770.7 $798.6 $746.2 $31.1 $24.5Pension and other postretirement benefits costs355.3 549.2 174.2 279.3 181.1 269.9AROs65.7 62.4 36.7 38.6 29.0 23.8Emission allowances30.0 30.0 30.0 30.0 — —Environmental-related costs25.0 34.9 21.0 30.3 4.0 4.6Derivatives21.1 40.2 5.9 16.3 15.2 23.9Debt redemption costs17.9 19.8 12.2 13.6 5.7 6.2IPL’s electric transmission service costs8.3 16.6 8.3 16.6 — —Other60.2 88.6 26.6 47.0 33.6 41.6 $1,413.2 $1,612.4 $1,113.5 $1,217.9 $299.7 $394.5A portion of the regulatory assets in the above table are not earning a return. These regulatory assets are expected to be recovered from customers in future rates; however, the respective carrying costs of these assets are not expected to be recovered from customers in future rates. At December 31, 2013, IPL and WPL had $39 million and $13 million, respectively, of regulatory assets representing past expenditures that were not earning a return. IPL’s regulatory assets that were not earning a return consisted primarily of debt redemption costs and electric transmission service costs. WPL’s regulatory assets that were not earning a return consisted primarily of amounts related to wholesale customer rate recovery, which is discussed in Note 1(f). The other regulatory assets reported in the above table either earn a return or the cash has not yet been expended, in which case the assets are offset by liabilities that also do not incur a carrying cost.Tax-related - IPL and WPL record regulatory assets for certain temporary differences (primarily related to utility property, plant and equipment at IPL) that result in a decrease in current rates charged to customers and an increase in future rates charged to customers based on the timing of income tax expense that is used to determine such rates. These temporary differences include the impacts of qualifying deductions for repairs expenditures and allocation of mixed service costs, and Iowa accelerated tax depreciation, which all contribute to lower current income tax expense during the first part of an asset’s useful life and higher current tax expense during the last part of an asset’s useful life. These regulatory assets will be recovered from customers in the future when these temporary differences reverse resulting in additional current income tax expense used to determine customers’ rates. During 2013, Alliant Energy’s and IPL’s “Tax-related” regulatory assets in the above table increased primarily due to qualifying repairs expenditure deductions at IPL.Pension and other postretirement benefits costs - The IUB and the PSCW have authorized IPL and WPL to record the retail portion of their respective previously unrecognized net actuarial gains and losses, and prior service costs and credits, as regulatory assets in lieu of AOCL on the Consolidated Balance Sheets, as these amounts are expected to be recovered in future rates. IPL and WPL also recognize the wholesale portion of their previously unrecognized net actuarial gains and losses, and prior service costs and credits, as regulatory assets on the Consolidated Balance Sheets because these costs are expected to be recovered in rates in future periods under the formula rate structure. These regulatory assets will be increased or decreased as the net actuarial gains or losses, and prior service costs or credits, are subsequently amortized and recognized as a component of net periodic benefit costs. Regulatory assets are also increased or decreased as a result of the annual defined benefit plan measurement process. During 2013, Alliant Energy’s, IPL’s and WPL’s pension and other postretirement benefits costs regulatory assets decreased due to a decrease in unrecognized net actuarial losses caused by higher discount rates and higher returns on assets compared to assumptions used in the annual defined benefit plan measurement process as of December 31, 2012.Pension and other postretirement benefits costs are included within the recoverable cost of service component of rates charged to IPL’s and WPL’s customers. The recoverable costs included in customers’ rates are based upon pension and other postretirement benefits costs determined in accordance with GAAP and are calculated using different methods for the various regulatory jurisdictions in which IPL and WPL operate. The methods for IPL’s and WPL’s primary regulatory jurisdictions are described below. The IUB authorized IPL in its most recent Iowa retail electric rate case order to recover from its retail electric customers in Iowa an allocated portion of annual costs equal to a two-year simple average of actual costs incurred during its test year (2009) and an estimate of costs for its forward-looking post-test year (2010). The PSCW authorized WPL in its most recent Wisconsin retail rate case to recover from its electric and gas retail customers in base rates an estimated allocated portion of annual costs equal to the costs expected to be incurred during the 2013 and 2014 test year period. WPL is authorized to recover from its wholesale customers an allocated portion of actual pension costs incurred each year. In accordance with FERC-approved formula rates, any over- or under-collection of these costs each year are refunded to or recovered from customers through subsequent changes to wholesale customer rates. WPL is authorized to recover from its wholesale customers an allocated portion of other postretirement benefits costs based on the amount of other postretirement benefits costs incurred in 2006. Refer to Note 12(a) for additional details regarding pension and other postretirement benefits costs.AROs - Alliant Energy, IPL and WPL believe it is probable that any differences between expenses accrued for legal AROs related to their regulated operations and expenses recovered currently in rates will be recoverable in future rates, and are deferring the differences as regulatory assets. Refer to Note 13 for additional details of AROs.Emission allowances - IPL entered into forward contracts in 2007 to purchase SO2 emission allowances with vintage years of 2014 through 2017 from various counterparties for $34 million to meet future CAIR emission reduction standards. Any SO2 emission allowances acquired under these forward contracts may be used to meet requirements under the existing Acid Rain program regulations or the more stringent CAIR emission reduction standards but are not eligible to be used for compliance requirements under CSAPR. In 2011, the EPA issued CSAPR to replace CAIR with an anticipated effective date in 2012. As a result of the issuance of CSAPR, Alliant Energy and IPL concluded in 2011 that the allowances to be acquired under these forward contracts would not be needed by IPL to comply with expected environmental regulations in the future. The value of these allowances was nominal, which was significantly below the $34 million contract price for these allowances. As a result, Alliant Energy and IPL recognized charges of $34 million for these forward contracts in 2011 with an offsetting obligation recorded in other long-term liabilities and deferred credits. Alliant Energy and IPL concluded that $30 million of the charges are probable of recovery from IPL’s customers, and therefore, were recorded to regulatory assets in 2011. The remaining $4 million of charges were determined not to be probable of recovery from IPL’s customers resulting in $2 million of charges related to electric customers recorded to “Electric production fuel and energy purchases” and $2 million of charges related to steam customers recorded to “Utility - Other operation and maintenance” in Alliant Energy’s and IPL’s Consolidated Statements of Income in 2011. In 2012, the D.C. Circuit Court vacated and remanded CSAPR for further revision to the EPA. The D.C. Circuit Court order also requires the EPA to continue administering CAIR pending the promulgation of a valid replacement for CSAPR. Despite CSAPR being vacated, the current value of these allowances continues to be nominal and significantly below the $34 million contract price for these allowances. Alliant Energy and IPL currently believe that CAIR will be replaced in the future, either by a modified CSAPR or another rule that addresses the interstate transport of air pollutants.Environmental-related costs - The IUB has permitted IPL to recover prudently incurred costs by allowing a representative level of MGP costs in the recoverable cost of service component of rates, as determined in its most recent retail gas rate case. Under the current rate-making treatment approved by the PSCW, the MGP expenditures of WPL are deferred and collected from retail gas customers over a five-year period after new rates are implemented. The MPUC allows the deferral of MGP-related costs applicable to IPL’s Minnesota sites and IPL has received approval to recover such costs in retail gas rates in Minnesota in its most recent retail gas rate case. Regulatory assets recorded by IPL and WPL reflect the probable future rate recovery of MGP expenditures. Refer to Note 16(e) for additional details of environmental-related MGP costs.Derivatives - In accordance with IPL’s and WPL’s fuel and natural gas recovery mechanisms, prudently incurred costs from derivative instruments are recovered from customers in the future after any losses are realized and gains from derivative instruments are refunded to customers in the future after any gains are realized. Based on these recovery mechanisms, the changes in the fair value of derivative liabilities/assets resulted in comparable changes to regulatory assets/liabilities on the Consolidated Balance Sheets. Refer to Note 15 for additional details of derivative assets and derivative liabilities.Debt redemption costs - For debt retired early with no subsequent re-issuance, IPL and WPL defer any debt repayment premiums and unamortized debt issuance costs and discounts as regulatory assets. These regulatory assets are amortized over the remaining original life of the debt retired early. Debt repayment premiums and other losses resulting from the refinancing of debt by IPL and WPL are deferred as regulatory assets and amortized over the life of the new debt issued.IPL’s electric transmission service costs - In 2010, IPL incurred electric transmission service costs billed by ITC under the Attachment “O” rate for ITC’s under-recovered 2008 costs. In 2010, the IUB issued an order authorizing IPL to defer the Iowa retail portion of these under-recovered costs and amortize the deferred costs over a five-year period ending December 2014. In accordance with this order, IPL is amortizing $8 million of this regulatory asset annually, with an equal and offsetting amount of amortization of IPL’s regulatory liability related to its electric transmission assets sale. The IUB determined that IPL should not include the unamortized balance of these deferred costs in electric rate base during the five-year recovery period.Other - Alliant Energy, IPL and WPL assess whether IPL’s and WPL’s regulatory assets are probable of future recovery by considering factors such as applicable regulations, recent orders by the applicable regulatory agencies, historical treatment of similar costs by the applicable regulatory agencies and regulatory environment changes. Based on these assessments, Alliant Energy, IPL and WPL believe the regulatory assets recognized as of December 31, 2013 in the above table are probable of future recovery. However, no assurance can be made that IPL and WPL will recover all of these regulatory assets in future rates. If future recovery of a regulatory asset ceases to be probable, the regulatory asset will be charged to expense in the period in which future recovery ceases to be probable.Based on the PSCW’s July 2012 order related to WPL’s 2013/2014 test period Wisconsin retail electric and gas rate case, WPL was authorized to recover previously incurred costs associated with the acquisition of a 25% ownership interest in Edgewater Unit 5 and proposed emission controls projects. As a result, Alliant Energy and WPL recorded a $5 million increase to regulatory assets, and a $5 million credit to “Utility - Other operation and maintenance” in their Consolidated Statements of Income in 2012.Based on assessments completed in 2011, Alliant Energy, IPL and WPL recognized impairment charges of $9 million, $2 million and $7 million, respectively, for regulatory assets that were no longer probable of future recovery. The regulatory asset impairment charges were recorded by Alliant Energy, IPL and WPL as reductions in regulatory assets and charges to “Utility - Other operation and maintenance” in their Consolidated Statements of Income in 2011.Based on the MPUC’s November 2011 order related to IPL’s 2009 test year Minnesota retail electric rate case, IPL was authorized to recover $2 million of previously incurred plant cancellation costs for Sutherland #4 over a 25-year period ending in 2037. As a result, Alliant Energy and IPL recorded a $2 million increase to regulatory assets, and a $2 million credit to “Utility - Other operation and maintenance” in their Consolidated Statements of Income in 2011.Regulatory Liabilities - At December 31, regulatory liabilities were comprised of the following items (in millions): Alliant Energy IPL WPL 2013 2012 2013 2012 2013 2012Cost of removal obligations$418.9 $408.7 $277.7 $268.0 $141.2 $140.7IPL’s tax benefit riders265.4 355.8 265.4 355.8 — —Energy conservation cost recovery52.7 55.1 9.3 10.0 43.4 45.1IPL’s electric transmission assets sale21.6 32.5 21.6 32.5 — —IPL’s electric transmission cost recovery14.6 — 14.6 — — —Commodity cost recovery7.5 17.7 5.5 5.2 2.0 12.5Other40.8 46.3 20.8 29.9 20.0 16.4 $821.5 $916.1 $614.9 $701.4 $206.6 $214.7Regulatory liabilities related to cost of removal obligations, to the extent expensed through depreciation rates, reduce rate base. A significant portion of the remaining regulatory liabilities are not used to reduce rate base in the revenue requirement calculations utilized in IPL’s and WPL’s respective rate proceedings.Cost of removal obligations - Alliant Energy, IPL and WPL collect in rates future removal costs for many assets that do not have associated legal AROs. Alliant Energy, IPL and WPL record a regulatory liability for the estimated amounts they have collected in rates for these future removal costs less amounts spent on removal activities.IPL’s tax benefit riders - At December 31, 2013, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities in the above table consisted of $228.9 million and $36.5 million for the electric and gas tax benefit riders, respectively. These regulatory liabilities decreased $90 million in aggregate in 2013 due to the following items:Electric tax benefit rider - In January 2011, the IUB approved an electric tax benefit rider proposed by IPL, which utilizes regulatory liabilities to credit bills of Iowa retail electric customers beginning in February 2011 to help offset the impact of rate increases on such customers. These regulatory liabilities are related to tax benefits from tax accounting method changes for repairs expenditures, allocation of mixed service costs and allocation of insurance proceeds from floods in 2008. Alliant Energy and IPL recognize an offsetting reduction to income tax expense for the after-tax amounts credited to IPL’s retail electric customers’ bills in Iowa, resulting in no impact to Alliant Energy’s and IPL’s net income from the electric tax benefit rider. In 2013, 2012 and 2011, Alliant Energy and IPL utilized $79 million, $83 million and $61 million, respectively, of electric tax benefit rider-related regulatory liabilities accumulated in prior years to credit IPL’s Iowa retail electric customers’ bills. In 2013, 2012 and 2011, the $79 million, $83 million and $61 million reductions to “Electric operating revenues” resulted in $33 million, $35 million and $25 million of credits to “Income tax expense (benefit)” as a result of the decrease in taxable income in Alliant Energy’s and IPL’s Consolidated Statements of Income in 2013, 2012 and 2011, respectively. In 2013, 2012 and 2011, additional reductions to “Income tax expense (benefit)” of $46 million, $48 million and $36 million, respectively, were also recognized in Alliant Energy’s and IPL’s Consolidated Statements of Income representing the tax benefits realized related to the electric tax benefit rider. In December 2013, the IUB issued an order authorizing $85 million of regulatory liabilities from tax benefits to be credited to IPL’s retail electric customers’ bills in Iowa during 2014 through the electric tax benefit rider.Gas tax benefit rider - In November 2012, the IUB approved a gas tax benefit rider proposed by IPL, which utilizes up to $12 million of regulatory liabilities annually to credit bills of Iowa retail gas customers beginning in January 2013 through December 2015 to help offset the impact of rate increases on such customers. These regulatory liabilities are related to tax benefits from tax accounting method changes for repairs expenditures, allocation of mixed service costs and allocation of insurance proceeds from floods in 2008. Any remaining benefit, including any portion not utilized of the agreed upon amount from January 2013 through December 2015, will be credited to Iowa’s retail gas customers’ bills in 2016. In 2013, Alliant Energy and IPL utilized $11 million of gas tax benefit rider-related regulatory liabilities to credit IPL’s Iowa retail gas customers’ bills. In 2013, the $11 million reduction to “Gas operating revenues” resulted in $4 million of credits to “Income tax expense (benefit)” as a result of the decrease in taxable income in Alliant Energy’s and IPL’s Consolidated Statements of Income in 2013. In 2013, additional reductions to “Income tax expense (benefit)” of $7 million were also recognized in Alliant Energy’s and IPL’s Consolidated Statements of Income representing the tax benefits realized related to the gas tax benefit rider.Refer to Note 11 for additional details regarding IPL’s tax benefit riders.Energy conservation cost recovery - WPL and IPL collect revenues from their customers to offset certain expenditures incurred by WPL and IPL for conservation programs, including state mandated programs and Shared Savings programs. Differences between forecasted costs used to set rates and actual costs for these programs are deferred as a regulatory asset or regulatory liability.IPL’s electric transmission assets sale - In 2007, IPL completed the sale of its electric transmission assets to ITC and recognized a gain based on the terms of the agreement. Upon closing of the sale, IPL established a regulatory liability of $89 million pursuant to conditions established by the IUB when it allowed the transaction to proceed. The regulatory liability represented the present value of IPL’s obligation to refund to its customers payments beginning in the year IPL’s customers experience an increase in rates related to the transmission charges assessed by ITC. The regulatory liability accrues interest at the monthly average U.S. Treasury rate for three-year maturities.Iowa retail portion - In 2009, the IUB issued an order authorizing IPL to use a portion of this regulatory liability to reduce Iowa retail electric customers’ rates by $12 million for the period from July 2009 through February 2010 with billing credits included in the monthly energy adjustment clause. In 2010, the IUB issued an order authorizing IPL to use a portion of this regulatory liability to offset electric transmission service costs expected to be billed to IPL by ITC in 2010 related to ITC’s 2008 transmission revenue adjustment. IPL expects to utilize $41 million of this regulatory liability over a five-year period ending December 2014 to offset the Iowa retail portion of transmission costs billed to IPL by ITC in 2010 related to ITC’s 2008 transmission revenue adjustment. As a result, IPL is amortizing $8 million of this regulatory liability annually, with an equal and offsetting amount of amortization for IPL’s regulatory asset related to electric transmission service costs.In accordance with the IUB’s 2011 order related to IPL’s 2009 test year Iowa retail electric rate case, IPL was authorized to utilize regulatory liabilities in 2011 to offset transmission service expenses related to the Iowa retail portion of 2009 under-recovered costs billed to IPL. As a result, Alliant Energy and IPL recorded a reduction of $19 million in regulatory liabilities, and a reduction of $19 million in “Electric transmission service” in their Consolidated Statements of Income in 2011. The IUB also authorized IPL to utilize $3 million of this regulatory liability in 2011 to reduce IPL’s Iowa retail electric rate base associated with the Whispering Willow - East wind project.Minnesota retail portion - In 2010, the MPUC issued an interim rate order authorizing IPL to use a portion of this regulatory liability to implement an alternative transaction adjustment through its energy adjustment clause resulting in annual credits to its Minnesota retail electric customers beginning in July 2010 to coincide with the effective date of the interim rate increase for Minnesota retail customers. The amounts of the annual credits are dependent upon the level of KWhs sold to IPL’s Minnesota retail customers. In accordance with the MPUC’s November 2011 order related to IPL’s 2009 test year Minnesota retail electric rate case, IPL was authorized to refund a higher amount of the gain realized from the sale of its electric transmission assets in 2007 to its Minnesota retail electric customers than previously estimated. As a result, Alliant Energy and IPL recorded a $5 million increase to regulatory liabilities, and a $5 million charge to “Utility - Other operation and maintenance” in their Consolidated Statements of Income in 2011 for the additional amount to be refunded.Refunds related to any remaining balance of IPL’s electric transmission assets sale regulatory liability are expected to be determined in future rate proceedings or as part of the proposed sale of Minnesota electric distribution assets.IPL’s electric transmission cost recovery - Refer to Note 1(g) for additional details of IPL’s electric transmission service cost recovery mechanism.Commodity cost recovery - Refer to Note 1(g) for additional details of IPL’s and WPL’s cost recovery mechanisms. Refer to “WPL’s Retail Fuel-related Rate Filing (2012 Test Year)” below for discussion of refunds made to WPL’s retail electric customers in 2013.Utility Rate Cases -WPL’s Wisconsin Retail Electric and Gas Rate Case (2013/2014 Test Period) - In July 2012, WPL received an order from the PSCW authorizing WPL to implement a decrease in annual base rates for WPL’s retail gas customers of $13 million effective January 1, 2013, followed by a freeze of such gas base rates through the end of 2014. The order also authorized WPL to maintain customer base rates for its retail electric customers at their current levels through the end of 2014. The order included provisions that require WPL to defer a portion of its earnings if its annual regulatory return on common equity exceeds certain levels during 2013 or 2014 and allows WPL to request a change in retail base rates during this period if its annual regulatory return on common equity falls below a certain level. As of December 31, 2013, Alliant Energy and WPL did not record any material deferred amounts for these provisions. Refer to “Regulatory Assets” above for discussion of regulatory-related credits recorded in 2012 as a result of the PSCW’s order authorizing WPL to recover previously incurred costs associated with the acquisition of a 25% interest in Edgewater Unit 5 and proposed emission controls projects.IPL’s Iowa Retail Gas Rate Case (2011 Test Year) - In May 2012, IPL filed a request with the IUB to increase annual rates for its Iowa retail gas customers based on a 2011 historical test year as adjusted for certain known and measurable changes occurring up to 12 months after the commencement of the proceeding. IPL’s request included a proposal to utilize regulatory liabilities to credit bills of Iowa retail gas customers to help mitigate the impact of the proposed final rate increase on such customers. IPL proposed to reduce customer bills utilizing a gas tax benefit rider over a three-year period by approximately $36 million in aggregate. In conjunction with the filing, IPL implemented an interim retail gas rate increase of $9 million, or approximately 3%, on an annual basis, effective June 4, 2012, without regulatory review and subject to refund pending determination of final rates from the request. In 2012, Alliant Energy and IPL recorded $5 million in gas revenues from IPL’s Iowa retail gas customers related to the interim retail gas rate increase. In November 2012, the IUB approved a settlement agreement between IPL, the OCA and the Iowa Consumers Coalition related to IPL’s request, resulting in a final increase in annual rates for IPL’s Iowa retail gas customers of $11 million, or approximately 4%, effective January 10, 2013. The parties and the IUB also agreed to IPL’s proposed gas tax benefit rider. In 2013, Alliant Energy and IPL recorded $11 million in gas revenues from IPL’s Iowa retail gas customers related to the final retail gas rate increase. Refer to “Regulatory Liabilities” above for additional details on IPL’s gas tax benefit rider.IPL’s Iowa Retail Electric Rate Case (2009 Test Year) - Electric Tax Benefit Rider - In February 2013, the IUB issued an order allowing IPL to recognize a revenue requirement adjustment of $24 million in 2013 related to certain tax benefits from tax accounting method changes. The revenue requirement adjustment is recognized through the energy adjustment clause as a reduction of the credits on IPL’s Iowa retail electric customers’ bills from the electric tax benefit rider. In 2013, Alliant Energy and IPL recognized $24 million of the revenue requirement adjustment resulting in increases to electric revenues in their Consolidated Statements of Income. In December 2013, the IUB authorized IPL to reduce the billing credits on customers’ bills by $15 million in 2014 from tax benefits for the electric tax benefit rider to recognize the revenue requirement impact of the changes in tax accounting methods.IPL’s Minnesota Retail Electric Rate Case (2009 Test Year) - In May 2010, IPL filed a request with the MPUC to increase annual rates for its Minnesota retail electric customers based on a 2009 historical test year as adjusted for certain known and measurable items at the time of the filing. The key drivers for the filing included recovery of investments in the Whispering Willow - East wind project and emission controls projects at Lansing Unit 4, and recovery of increased electric transmission service costs. In conjunction with the filing, IPL implemented an interim retail rate increase of $14 million, on an annual basis, effective July 6, 2010. In November 2011, IPL received an order from the MPUC authorizing a final annual retail electric rate increase equivalent to $11 million. The final annual retail electric rate increase of $11 million includes $8 million of higher base rates, $2 million from the temporary renewable energy rider and $1 million from the utilization of regulatory liabilities to offset higher electric transmission service costs. Refer to “Regulatory Assets” and “Regulatory Liabilities” above for discussion of changes to regulatory assets and regulatory liabilities in 2011 based on the MPUC’s decisions in this rate case. Refer to Note 3(a) for discussion of adjustments made by Alliant Energy and IPL in 2011 and 2013 to the carrying value of IPL’s Whispering Willow - East wind project, based on amounts IPL determined were probable of being disallowed for recovery from its Minnesota retail electric customers.WPL’s Retail Fuel-related Rate Filing (2014 Test Year) - In December 2013, WPL received an order from the PSCW authorizing an annual retail electric rate increase of $19 million, or approximately 2%, effective January 1, 2014 to reflect anticipated increases in retail fuel-related costs in 2014 compared to the fuel-related cost estimates used to determine rates for 2013. WPL’s 2014 fuel-related costs will be subject to deferral if they fall outside an annual bandwidth of plus or minus 2% of the approved annual forecasted fuel-related costs.WPL’s Retail Fuel-related Rate Filing (2013 Test Year) - In December 2012, WPL received an order from the PSCW authorizing an annual retail electric rate decrease of $29 million, or approximately 3%, effective January 1, 2013 to reflect anticipated decreases in retail fuel-related costs in 2013 compared to the fuel-related cost estimates used to determine rates for 2012. WPL’s 2013 fuel-related costs were subject to deferral if they fell outside an annual bandwidth of plus or minus 2% of the approved annual forecasted fuel-related costs. Retail fuel-related costs incurred by WPL for 2013 did not fall outside of the fuel monitoring range.WPL’s Retail Fuel-related Rate Filing (2012 Test Year) - In December 2011, WPL received an order from the PSCW authorizing an annual retail electric rate increase of $4 million, effective January 1, 2012 to reflect anticipated increases in retail fuel-related costs in 2012 compared to the fuel-related cost estimates used to determine rates for 2011. The 2012 fuel-related costs were subject to an annual bandwidth of plus or minus 2%. Retail fuel-related costs incurred by WPL in 2012 were lower than retail fuel-related costs used to determine rates for such period resulting in an over-collection of fuel-related costs for 2012 of approximately $17 million (including $11 million outside the approved range for 2012 recorded in “Regulatory liabilities” on Alliant Energy’s and WPL’s Consolidated Balance Sheets as of December 31, 2012). In August 2013, WPL received an order from the PSCW to refund $12 million, including interest, to its retail electric customers for these over-collections, which WPL completed in September 2013.Refer to Note 1(g) for further discussion of WPL’s fuel cost recovery mechanism.
WPL [Member]
Regulatory Matters [Line Items]
Regulatory Matters	REGULATORY MATTERSRegulatory Assets - At December 31, regulatory assets were comprised of the following items (in millions): Alliant Energy IPL WPL 2013 2012 2013 2012 2013 2012Tax-related$829.7 $770.7 $798.6 $746.2 $31.1 $24.5Pension and other postretirement benefits costs355.3 549.2 174.2 279.3 181.1 269.9AROs65.7 62.4 36.7 38.6 29.0 23.8Emission allowances30.0 30.0 30.0 30.0 — —Environmental-related costs25.0 34.9 21.0 30.3 4.0 4.6Derivatives21.1 40.2 5.9 16.3 15.2 23.9Debt redemption costs17.9 19.8 12.2 13.6 5.7 6.2IPL’s electric transmission service costs8.3 16.6 8.3 16.6 — —Other60.2 88.6 26.6 47.0 33.6 41.6 $1,413.2 $1,612.4 $1,113.5 $1,217.9 $299.7 $394.5A portion of the regulatory assets in the above table are not earning a return. These regulatory assets are expected to be recovered from customers in future rates; however, the respective carrying costs of these assets are not expected to be recovered from customers in future rates. At December 31, 2013, IPL and WPL had $39 million and $13 million, respectively, of regulatory assets representing past expenditures that were not earning a return. IPL’s regulatory assets that were not earning a return consisted primarily of debt redemption costs and electric transmission service costs. WPL’s regulatory assets that were not earning a return consisted primarily of amounts related to wholesale customer rate recovery, which is discussed in Note 1(f). The other regulatory assets reported in the above table either earn a return or the cash has not yet been expended, in which case the assets are offset by liabilities that also do not incur a carrying cost.Tax-related - IPL and WPL record regulatory assets for certain temporary differences (primarily related to utility property, plant and equipment at IPL) that result in a decrease in current rates charged to customers and an increase in future rates charged to customers based on the timing of income tax expense that is used to determine such rates. These temporary differences include the impacts of qualifying deductions for repairs expenditures and allocation of mixed service costs, and Iowa accelerated tax depreciation, which all contribute to lower current income tax expense during the first part of an asset’s useful life and higher current tax expense during the last part of an asset’s useful life. These regulatory assets will be recovered from customers in the future when these temporary differences reverse resulting in additional current income tax expense used to determine customers’ rates. During 2013, Alliant Energy’s and IPL’s “Tax-related” regulatory assets in the above table increased primarily due to qualifying repairs expenditure deductions at IPL.Pension and other postretirement benefits costs - The IUB and the PSCW have authorized IPL and WPL to record the retail portion of their respective previously unrecognized net actuarial gains and losses, and prior service costs and credits, as regulatory assets in lieu of AOCL on the Consolidated Balance Sheets, as these amounts are expected to be recovered in future rates. IPL and WPL also recognize the wholesale portion of their previously unrecognized net actuarial gains and losses, and prior service costs and credits, as regulatory assets on the Consolidated Balance Sheets because these costs are expected to be recovered in rates in future periods under the formula rate structure. These regulatory assets will be increased or decreased as the net actuarial gains or losses, and prior service costs or credits, are subsequently amortized and recognized as a component of net periodic benefit costs. Regulatory assets are also increased or decreased as a result of the annual defined benefit plan measurement process. During 2013, Alliant Energy’s, IPL’s and WPL’s pension and other postretirement benefits costs regulatory assets decreased due to a decrease in unrecognized net actuarial losses caused by higher discount rates and higher returns on assets compared to assumptions used in the annual defined benefit plan measurement process as of December 31, 2012.Pension and other postretirement benefits costs are included within the recoverable cost of service component of rates charged to IPL’s and WPL’s customers. The recoverable costs included in customers’ rates are based upon pension and other postretirement benefits costs determined in accordance with GAAP and are calculated using different methods for the various regulatory jurisdictions in which IPL and WPL operate. The methods for IPL’s and WPL’s primary regulatory jurisdictions are described below. The IUB authorized IPL in its most recent Iowa retail electric rate case order to recover from its retail electric customers in Iowa an allocated portion of annual costs equal to a two-year simple average of actual costs incurred during its test year (2009) and an estimate of costs for its forward-looking post-test year (2010). The PSCW authorized WPL in its most recent Wisconsin retail rate case to recover from its electric and gas retail customers in base rates an estimated allocated portion of annual costs equal to the costs expected to be incurred during the 2013 and 2014 test year period. WPL is authorized to recover from its wholesale customers an allocated portion of actual pension costs incurred each year. In accordance with FERC-approved formula rates, any over- or under-collection of these costs each year are refunded to or recovered from customers through subsequent changes to wholesale customer rates. WPL is authorized to recover from its wholesale customers an allocated portion of other postretirement benefits costs based on the amount of other postretirement benefits costs incurred in 2006. Refer to Note 12(a) for additional details regarding pension and other postretirement benefits costs.AROs - Alliant Energy, IPL and WPL believe it is probable that any differences between expenses accrued for legal AROs related to their regulated operations and expenses recovered currently in rates will be recoverable in future rates, and are deferring the differences as regulatory assets. Refer to Note 13 for additional details of AROs.Emission allowances - IPL entered into forward contracts in 2007 to purchase SO2 emission allowances with vintage years of 2014 through 2017 from various counterparties for $34 million to meet future CAIR emission reduction standards. Any SO2 emission allowances acquired under these forward contracts may be used to meet requirements under the existing Acid Rain program regulations or the more stringent CAIR emission reduction standards but are not eligible to be used for compliance requirements under CSAPR. In 2011, the EPA issued CSAPR to replace CAIR with an anticipated effective date in 2012. As a result of the issuance of CSAPR, Alliant Energy and IPL concluded in 2011 that the allowances to be acquired under these forward contracts would not be needed by IPL to comply with expected environmental regulations in the future. The value of these allowances was nominal, which was significantly below the $34 million contract price for these allowances. As a result, Alliant Energy and IPL recognized charges of $34 million for these forward contracts in 2011 with an offsetting obligation recorded in other long-term liabilities and deferred credits. Alliant Energy and IPL concluded that $30 million of the charges are probable of recovery from IPL’s customers, and therefore, were recorded to regulatory assets in 2011. The remaining $4 million of charges were determined not to be probable of recovery from IPL’s customers resulting in $2 million of charges related to electric customers recorded to “Electric production fuel and energy purchases” and $2 million of charges related to steam customers recorded to “Utility - Other operation and maintenance” in Alliant Energy’s and IPL’s Consolidated Statements of Income in 2011. In 2012, the D.C. Circuit Court vacated and remanded CSAPR for further revision to the EPA. The D.C. Circuit Court order also requires the EPA to continue administering CAIR pending the promulgation of a valid replacement for CSAPR. Despite CSAPR being vacated, the current value of these allowances continues to be nominal and significantly below the $34 million contract price for these allowances. Alliant Energy and IPL currently believe that CAIR will be replaced in the future, either by a modified CSAPR or another rule that addresses the interstate transport of air pollutants.Environmental-related costs - The IUB has permitted IPL to recover prudently incurred costs by allowing a representative level of MGP costs in the recoverable cost of service component of rates, as determined in its most recent retail gas rate case. Under the current rate-making treatment approved by the PSCW, the MGP expenditures of WPL are deferred and collected from retail gas customers over a five-year period after new rates are implemented. The MPUC allows the deferral of MGP-related costs applicable to IPL’s Minnesota sites and IPL has received approval to recover such costs in retail gas rates in Minnesota in its most recent retail gas rate case. Regulatory assets recorded by IPL and WPL reflect the probable future rate recovery of MGP expenditures. Refer to Note 16(e) for additional details of environmental-related MGP costs.Derivatives - In accordance with IPL’s and WPL’s fuel and natural gas recovery mechanisms, prudently incurred costs from derivative instruments are recovered from customers in the future after any losses are realized and gains from derivative instruments are refunded to customers in the future after any gains are realized. Based on these recovery mechanisms, the changes in the fair value of derivative liabilities/assets resulted in comparable changes to regulatory assets/liabilities on the Consolidated Balance Sheets. Refer to Note 15 for additional details of derivative assets and derivative liabilities.Debt redemption costs - For debt retired early with no subsequent re-issuance, IPL and WPL defer any debt repayment premiums and unamortized debt issuance costs and discounts as regulatory assets. These regulatory assets are amortized over the remaining original life of the debt retired early. Debt repayment premiums and other losses resulting from the refinancing of debt by IPL and WPL are deferred as regulatory assets and amortized over the life of the new debt issued.IPL’s electric transmission service costs - In 2010, IPL incurred electric transmission service costs billed by ITC under the Attachment “O” rate for ITC’s under-recovered 2008 costs. In 2010, the IUB issued an order authorizing IPL to defer the Iowa retail portion of these under-recovered costs and amortize the deferred costs over a five-year period ending December 2014. In accordance with this order, IPL is amortizing $8 million of this regulatory asset annually, with an equal and offsetting amount of amortization of IPL’s regulatory liability related to its electric transmission assets sale. The IUB determined that IPL should not include the unamortized balance of these deferred costs in electric rate base during the five-year recovery period.Other - Alliant Energy, IPL and WPL assess whether IPL’s and WPL’s regulatory assets are probable of future recovery by considering factors such as applicable regulations, recent orders by the applicable regulatory agencies, historical treatment of similar costs by the applicable regulatory agencies and regulatory environment changes. Based on these assessments, Alliant Energy, IPL and WPL believe the regulatory assets recognized as of December 31, 2013 in the above table are probable of future recovery. However, no assurance can be made that IPL and WPL will recover all of these regulatory assets in future rates. If future recovery of a regulatory asset ceases to be probable, the regulatory asset will be charged to expense in the period in which future recovery ceases to be probable.Based on the PSCW’s July 2012 order related to WPL’s 2013/2014 test period Wisconsin retail electric and gas rate case, WPL was authorized to recover previously incurred costs associated with the acquisition of a 25% ownership interest in Edgewater Unit 5 and proposed emission controls projects. As a result, Alliant Energy and WPL recorded a $5 million increase to regulatory assets, and a $5 million credit to “Utility - Other operation and maintenance” in their Consolidated Statements of Income in 2012.Based on assessments completed in 2011, Alliant Energy, IPL and WPL recognized impairment charges of $9 million, $2 million and $7 million, respectively, for regulatory assets that were no longer probable of future recovery. The regulatory asset impairment charges were recorded by Alliant Energy, IPL and WPL as reductions in regulatory assets and charges to “Utility - Other operation and maintenance” in their Consolidated Statements of Income in 2011.Based on the MPUC’s November 2011 order related to IPL’s 2009 test year Minnesota retail electric rate case, IPL was authorized to recover $2 million of previously incurred plant cancellation costs for Sutherland #4 over a 25-year period ending in 2037. As a result, Alliant Energy and IPL recorded a $2 million increase to regulatory assets, and a $2 million credit to “Utility - Other operation and maintenance” in their Consolidated Statements of Income in 2011.Regulatory Liabilities - At December 31, regulatory liabilities were comprised of the following items (in millions): Alliant Energy IPL WPL 2013 2012 2013 2012 2013 2012Cost of removal obligations$418.9 $408.7 $277.7 $268.0 $141.2 $140.7IPL’s tax benefit riders265.4 355.8 265.4 355.8 — —Energy conservation cost recovery52.7 55.1 9.3 10.0 43.4 45.1IPL’s electric transmission assets sale21.6 32.5 21.6 32.5 — —IPL’s electric transmission cost recovery14.6 — 14.6 — — —Commodity cost recovery7.5 17.7 5.5 5.2 2.0 12.5Other40.8 46.3 20.8 29.9 20.0 16.4 $821.5 $916.1 $614.9 $701.4 $206.6 $214.7Regulatory liabilities related to cost of removal obligations, to the extent expensed through depreciation rates, reduce rate base. A significant portion of the remaining regulatory liabilities are not used to reduce rate base in the revenue requirement calculations utilized in IPL’s and WPL’s respective rate proceedings.Cost of removal obligations - Alliant Energy, IPL and WPL collect in rates future removal costs for many assets that do not have associated legal AROs. Alliant Energy, IPL and WPL record a regulatory liability for the estimated amounts they have collected in rates for these future removal costs less amounts spent on removal activities.IPL’s tax benefit riders - At December 31, 2013, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities in the above table consisted of $228.9 million and $36.5 million for the electric and gas tax benefit riders, respectively. These regulatory liabilities decreased $90 million in aggregate in 2013 due to the following items:Electric tax benefit rider - In January 2011, the IUB approved an electric tax benefit rider proposed by IPL, which utilizes regulatory liabilities to credit bills of Iowa retail electric customers beginning in February 2011 to help offset the impact of rate increases on such customers. These regulatory liabilities are related to tax benefits from tax accounting method changes for repairs expenditures, allocation of mixed service costs and allocation of insurance proceeds from floods in 2008. Alliant Energy and IPL recognize an offsetting reduction to income tax expense for the after-tax amounts credited to IPL’s retail electric customers’ bills in Iowa, resulting in no impact to Alliant Energy’s and IPL’s net income from the electric tax benefit rider. In 2013, 2012 and 2011, Alliant Energy and IPL utilized $79 million, $83 million and $61 million, respectively, of electric tax benefit rider-related regulatory liabilities accumulated in prior years to credit IPL’s Iowa retail electric customers’ bills. In 2013, 2012 and 2011, the $79 million, $83 million and $61 million reductions to “Electric operating revenues” resulted in $33 million, $35 million and $25 million of credits to “Income tax expense (benefit)” as a result of the decrease in taxable income in Alliant Energy’s and IPL’s Consolidated Statements of Income in 2013, 2012 and 2011, respectively. In 2013, 2012 and 2011, additional reductions to “Income tax expense (benefit)” of $46 million, $48 million and $36 million, respectively, were also recognized in Alliant Energy’s and IPL’s Consolidated Statements of Income representing the tax benefits realized related to the electric tax benefit rider. In December 2013, the IUB issued an order authorizing $85 million of regulatory liabilities from tax benefits to be credited to IPL’s retail electric customers’ bills in Iowa during 2014 through the electric tax benefit rider.Gas tax benefit rider - In November 2012, the IUB approved a gas tax benefit rider proposed by IPL, which utilizes up to $12 million of regulatory liabilities annually to credit bills of Iowa retail gas customers beginning in January 2013 through December 2015 to help offset the impact of rate increases on such customers. These regulatory liabilities are related to tax benefits from tax accounting method changes for repairs expenditures, allocation of mixed service costs and allocation of insurance proceeds from floods in 2008. Any remaining benefit, including any portion not utilized of the agreed upon amount from January 2013 through December 2015, will be credited to Iowa’s retail gas customers’ bills in 2016. In 2013, Alliant Energy and IPL utilized $11 million of gas tax benefit rider-related regulatory liabilities to credit IPL’s Iowa retail gas customers’ bills. In 2013, the $11 million reduction to “Gas operating revenues” resulted in $4 million of credits to “Income tax expense (benefit)” as a result of the decrease in taxable income in Alliant Energy’s and IPL’s Consolidated Statements of Income in 2013. In 2013, additional reductions to “Income tax expense (benefit)” of $7 million were also recognized in Alliant Energy’s and IPL’s Consolidated Statements of Income representing the tax benefits realized related to the gas tax benefit rider.Refer to Note 11 for additional details regarding IPL’s tax benefit riders.Energy conservation cost recovery - WPL and IPL collect revenues from their customers to offset certain expenditures incurred by WPL and IPL for conservation programs, including state mandated programs and Shared Savings programs. Differences between forecasted costs used to set rates and actual costs for these programs are deferred as a regulatory asset or regulatory liability.IPL’s electric transmission assets sale - In 2007, IPL completed the sale of its electric transmission assets to ITC and recognized a gain based on the terms of the agreement. Upon closing of the sale, IPL established a regulatory liability of $89 million pursuant to conditions established by the IUB when it allowed the transaction to proceed. The regulatory liability represented the present value of IPL’s obligation to refund to its customers payments beginning in the year IPL’s customers experience an increase in rates related to the transmission charges assessed by ITC. The regulatory liability accrues interest at the monthly average U.S. Treasury rate for three-year maturities.Iowa retail portion - In 2009, the IUB issued an order authorizing IPL to use a portion of this regulatory liability to reduce Iowa retail electric customers’ rates by $12 million for the period from July 2009 through February 2010 with billing credits included in the monthly energy adjustment clause. In 2010, the IUB issued an order authorizing IPL to use a portion of this regulatory liability to offset electric transmission service costs expected to be billed to IPL by ITC in 2010 related to ITC’s 2008 transmission revenue adjustment. IPL expects to utilize $41 million of this regulatory liability over a five-year period ending December 2014 to offset the Iowa retail portion of transmission costs billed to IPL by ITC in 2010 related to ITC’s 2008 transmission revenue adjustment. As a result, IPL is amortizing $8 million of this regulatory liability annually, with an equal and offsetting amount of amortization for IPL’s regulatory asset related to electric transmission service costs.In accordance with the IUB’s 2011 order related to IPL’s 2009 test year Iowa retail electric rate case, IPL was authorized to utilize regulatory liabilities in 2011 to offset transmission service expenses related to the Iowa retail portion of 2009 under-recovered costs billed to IPL. As a result, Alliant Energy and IPL recorded a reduction of $19 million in regulatory liabilities, and a reduction of $19 million in “Electric transmission service” in their Consolidated Statements of Income in 2011. The IUB also authorized IPL to utilize $3 million of this regulatory liability in 2011 to reduce IPL’s Iowa retail electric rate base associated with the Whispering Willow - East wind project.Minnesota retail portion - In 2010, the MPUC issued an interim rate order authorizing IPL to use a portion of this regulatory liability to implement an alternative transaction adjustment through its energy adjustment clause resulting in annual credits to its Minnesota retail electric customers beginning in July 2010 to coincide with the effective date of the interim rate increase for Minnesota retail customers. The amounts of the annual credits are dependent upon the level of KWhs sold to IPL’s Minnesota retail customers. In accordance with the MPUC’s November 2011 order related to IPL’s 2009 test year Minnesota retail electric rate case, IPL was authorized to refund a higher amount of the gain realized from the sale of its electric transmission assets in 2007 to its Minnesota retail electric customers than previously estimated. As a result, Alliant Energy and IPL recorded a $5 million increase to regulatory liabilities, and a $5 million charge to “Utility - Other operation and maintenance” in their Consolidated Statements of Income in 2011 for the additional amount to be refunded.Refunds related to any remaining balance of IPL’s electric transmission assets sale regulatory liability are expected to be determined in future rate proceedings or as part of the proposed sale of Minnesota electric distribution assets.IPL’s electric transmission cost recovery - Refer to Note 1(g) for additional details of IPL’s electric transmission service cost recovery mechanism.Commodity cost recovery - Refer to Note 1(g) for additional details of IPL’s and WPL’s cost recovery mechanisms. Refer to “WPL’s Retail Fuel-related Rate Filing (2012 Test Year)” below for discussion of refunds made to WPL’s retail electric customers in 2013.Utility Rate Cases -WPL’s Wisconsin Retail Electric and Gas Rate Case (2013/2014 Test Period) - In July 2012, WPL received an order from the PSCW authorizing WPL to implement a decrease in annual base rates for WPL’s retail gas customers of $13 million effective January 1, 2013, followed by a freeze of such gas base rates through the end of 2014. The order also authorized WPL to maintain customer base rates for its retail electric customers at their current levels through the end of 2014. The order included provisions that require WPL to defer a portion of its earnings if its annual regulatory return on common equity exceeds certain levels during 2013 or 2014 and allows WPL to request a change in retail base rates during this period if its annual regulatory return on common equity falls below a certain level. As of December 31, 2013, Alliant Energy and WPL did not record any material deferred amounts for these provisions. Refer to “Regulatory Assets” above for discussion of regulatory-related credits recorded in 2012 as a result of the PSCW’s order authorizing WPL to recover previously incurred costs associated with the acquisition of a 25% interest in Edgewater Unit 5 and proposed emission controls projects.IPL’s Iowa Retail Gas Rate Case (2011 Test Year) - In May 2012, IPL filed a request with the IUB to increase annual rates for its Iowa retail gas customers based on a 2011 historical test year as adjusted for certain known and measurable changes occurring up to 12 months after the commencement of the proceeding. IPL’s request included a proposal to utilize regulatory liabilities to credit bills of Iowa retail gas customers to help mitigate the impact of the proposed final rate increase on such customers. IPL proposed to reduce customer bills utilizing a gas tax benefit rider over a three-year period by approximately $36 million in aggregate. In conjunction with the filing, IPL implemented an interim retail gas rate increase of $9 million, or approximately 3%, on an annual basis, effective June 4, 2012, without regulatory review and subject to refund pending determination of final rates from the request. In 2012, Alliant Energy and IPL recorded $5 million in gas revenues from IPL’s Iowa retail gas customers related to the interim retail gas rate increase. In November 2012, the IUB approved a settlement agreement between IPL, the OCA and the Iowa Consumers Coalition related to IPL’s request, resulting in a final increase in annual rates for IPL’s Iowa retail gas customers of $11 million, or approximately 4%, effective January 10, 2013. The parties and the IUB also agreed to IPL’s proposed gas tax benefit rider. In 2013, Alliant Energy and IPL recorded $11 million in gas revenues from IPL’s Iowa retail gas customers related to the final retail gas rate increase. Refer to “Regulatory Liabilities” above for additional details on IPL’s gas tax benefit rider.IPL’s Iowa Retail Electric Rate Case (2009 Test Year) - Electric Tax Benefit Rider - In February 2013, the IUB issued an order allowing IPL to recognize a revenue requirement adjustment of $24 million in 2013 related to certain tax benefits from tax accounting method changes. The revenue requirement adjustment is recognized through the energy adjustment clause as a reduction of the credits on IPL’s Iowa retail electric customers’ bills from the electric tax benefit rider. In 2013, Alliant Energy and IPL recognized $24 million of the revenue requirement adjustment resulting in increases to electric revenues in their Consolidated Statements of Income. In December 2013, the IUB authorized IPL to reduce the billing credits on customers’ bills by $15 million in 2014 from tax benefits for the electric tax benefit rider to recognize the revenue requirement impact of the changes in tax accounting methods.IPL’s Minnesota Retail Electric Rate Case (2009 Test Year) - In May 2010, IPL filed a request with the MPUC to increase annual rates for its Minnesota retail electric customers based on a 2009 historical test year as adjusted for certain known and measurable items at the time of the filing. The key drivers for the filing included recovery of investments in the Whispering Willow - East wind project and emission controls projects at Lansing Unit 4, and recovery of increased electric transmission service costs. In conjunction with the filing, IPL implemented an interim retail rate increase of $14 million, on an annual basis, effective July 6, 2010. In November 2011, IPL received an order from the MPUC authorizing a final annual retail electric rate increase equivalent to $11 million. The final annual retail electric rate increase of $11 million includes $8 million of higher base rates, $2 million from the temporary renewable energy rider and $1 million from the utilization of regulatory liabilities to offset higher electric transmission service costs. Refer to “Regulatory Assets” and “Regulatory Liabilities” above for discussion of changes to regulatory assets and regulatory liabilities in 2011 based on the MPUC’s decisions in this rate case. Refer to Note 3(a) for discussion of adjustments made by Alliant Energy and IPL in 2011 and 2013 to the carrying value of IPL’s Whispering Willow - East wind project, based on amounts IPL determined were probable of being disallowed for recovery from its Minnesota retail electric customers.WPL’s Retail Fuel-related Rate Filing (2014 Test Year) - In December 2013, WPL received an order from the PSCW authorizing an annual retail electric rate increase of $19 million, or approximately 2%, effective January 1, 2014 to reflect anticipated increases in retail fuel-related costs in 2014 compared to the fuel-related cost estimates used to determine rates for 2013. WPL’s 2014 fuel-related costs will be subject to deferral if they fall outside an annual bandwidth of plus or minus 2% of the approved annual forecasted fuel-related costs.WPL’s Retail Fuel-related Rate Filing (2013 Test Year) - In December 2012, WPL received an order from the PSCW authorizing an annual retail electric rate decrease of $29 million, or approximately 3%, effective January 1, 2013 to reflect anticipated decreases in retail fuel-related costs in 2013 compared to the fuel-related cost estimates used to determine rates for 2012. WPL’s 2013 fuel-related costs were subject to deferral if they fell outside an annual bandwidth of plus or minus 2% of the approved annual forecasted fuel-related costs. Retail fuel-related costs incurred by WPL for 2013 did not fall outside of the fuel monitoring range.WPL’s Retail Fuel-related Rate Filing (2012 Test Year) - In December 2011, WPL received an order from the PSCW authorizing an annual retail electric rate increase of $4 million, effective January 1, 2012 to reflect anticipated increases in retail fuel-related costs in 2012 compared to the fuel-related cost estimates used to determine rates for 2011. The 2012 fuel-related costs were subject to an annual bandwidth of plus or minus 2%. Retail fuel-related costs incurred by WPL in 2012 were lower than retail fuel-related costs used to determine rates for such period resulting in an over-collection of fuel-related costs for 2012 of approximately $17 million (including $11 million outside the approved range for 2012 recorded in “Regulatory liabilities” on Alliant Energy’s and WPL’s Consolidated Balance Sheets as of December 31, 2012). In August 2013, WPL received an order from the PSCW to refund $12 million, including interest, to its retail electric customers for these over-collections, which WPL completed in September 2013.Refer to Note 1(g) for further discussion of WPL’s fuel cost recovery mechanism.

REGULATORY MATTERSRegulatory Assets - At December 31, regulatory assets were comprised of the following items (in millions): Alliant Energy IPL WPL 2013 2012 2013 2012 2013 2012Tax-related$829.7 $770.7 $798.6 $746.2 $31.1 $24.5Pension and other postretirement benefits costs355.3 549.2 174.2 279.3 181.1 269.9AROs65.7 62.4 36.7 38.6 29.0 23.8Emission allowances30.0 30.0 30.0 30.0 — —Environmental-related costs25.0 34.9 21.0 30.3 4.0 4.6Derivatives21.1 40.2 5.9 16.3 15.2 23.9Debt redemption costs17.9 19.8 12.2 13.6 5.7 6.2IPL’s electric transmission service costs8.3 16.6 8.3 16.6 — —Other60.2 88.6 26.6 47.0 33.6 41.6 $1,413.2 $1,612.4 $1,113.5 $1,217.9 $299.7 $394.5A portion of the regulatory assets in the above table are not earning a return. These regulatory assets are expected to be recovered from customers in future rates; however, the respective carrying costs of these assets are not expected to be recovered from customers in future rates. At December 31, 2013, IPL and WPL had $39 million and $13 million, respectively, of regulatory assets representing past expenditures that were not earning a return. IPL’s regulatory assets that were not earning a return consisted primarily of debt redemption costs and electric transmission service costs. WPL’s regulatory assets that were not earning a return consisted primarily of amounts related to wholesale customer rate recovery, which is discussed in Note 1(f). The other regulatory assets reported in the above table either earn a return or the cash has not yet been expended, in which case the assets are offset by liabilities that also do not incur a carrying cost.Tax-related - IPL and WPL record regulatory assets for certain temporary differences (primarily related to utility property, plant and equipment at IPL) that result in a decrease in current rates charged to customers and an increase in future rates charged to customers based on the timing of income tax expense that is used to determine such rates. These temporary differences include the impacts of qualifying deductions for repairs expenditures and allocation of mixed service costs, and Iowa accelerated tax depreciation, which all contribute to lower current income tax expense during the first part of an asset’s useful life and higher current tax expense during the last part of an asset’s useful life. These regulatory assets will be recovered from customers in the future when these temporary differences reverse resulting in additional current income tax expense used to determine customers’ rates. During 2013, Alliant Energy’s and IPL’s “Tax-related” regulatory assets in the above table increased primarily due to qualifying repairs expenditure deductions at IPL.Pension and other postretirement benefits costs - The IUB and the PSCW have authorized IPL and WPL to record the retail portion of their respective previously unrecognized net actuarial gains and losses, and prior service costs and credits, as regulatory assets in lieu of AOCL on the Consolidated Balance Sheets, as these amounts are expected to be recovered in future rates. IPL and WPL also recognize the wholesale portion of their previously unrecognized net actuarial gains and losses, and prior service costs and credits, as regulatory assets on the Consolidated Balance Sheets because these costs are expected to be recovered in rates in future periods under the formula rate structure. These regulatory assets will be increased or decreased as the net actuarial gains or losses, and prior service costs or credits, are subsequently amortized and recognized as a component of net periodic benefit costs. Regulatory assets are also increased or decreased as a result of the annual defined benefit plan measurement process. During 2013, Alliant Energy’s, IPL’s and WPL’s pension and other postretirement benefits costs regulatory assets decreased due to a decrease in unrecognized net actuarial losses caused by higher discount rates and higher returns on assets compared to assumptions used in the annual defined benefit plan measurement process as of December 31, 2012.Pension and other postretirement benefits costs are included within the recoverable cost of service component of rates charged to IPL’s and WPL’s customers. The recoverable costs included in customers’ rates are based upon pension and other postretirement benefits costs determined in accordance with GAAP and are calculated using different methods for the various regulatory jurisdictions in which IPL and WPL operate. The methods for IPL’s and WPL’s primary regulatory jurisdictions are described below. The IUB authorized IPL in its most recent Iowa retail electric rate case order to recover from its retail electric customers in Iowa an allocated portion of annual costs equal to a two-year simple average of actual costs incurred during its test year (2009) and an estimate of costs for its forward-looking post-test year (2010). The PSCW authorized WPL in its most recent Wisconsin retail rate case to recover from its electric and gas retail customers in base rates an estimated allocated portion of annual costs equal to the costs expected to be incurred during the 2013 and 2014 test year period. WPL is authorized to recover from its wholesale customers an allocated portion of actual pension costs incurred each year. In accordance with FERC-approved formula rates, any over- or under-collection of these costs each year are refunded to or recovered from customers through subsequent changes to wholesale customer rates. WPL is authorized to recover from its wholesale customers an allocated portion of other postretirement benefits costs based on the amount of other postretirement benefits costs incurred in 2006. Refer to Note 12(a) for additional details regarding pension and other postretirement benefits costs.AROs - Alliant Energy, IPL and WPL believe it is probable that any differences between expenses accrued for legal AROs related to their regulated operations and expenses recovered currently in rates will be recoverable in future rates, and are deferring the differences as regulatory assets. Refer to Note 13 for additional details of AROs.Emission allowances - IPL entered into forward contracts in 2007 to purchase SO2 emission allowances with vintage years of 2014 through 2017 from various counterparties for $34 million to meet future CAIR emission reduction standards. Any SO2 emission allowances acquired under these forward contracts may be used to meet requirements under the existing Acid Rain program regulations or the more stringent CAIR emission reduction standards but are not eligible to be used for compliance requirements under CSAPR. In 2011, the EPA issued CSAPR to replace CAIR with an anticipated effective date in 2012. As a result of the issuance of CSAPR, Alliant Energy and IPL concluded in 2011 that the allowances to be acquired under these forward contracts would not be needed by IPL to comply with expected environmental regulations in the future. The value of these allowances was nominal, which was significantly below the $34 million contract price for these allowances. As a result, Alliant Energy and IPL recognized charges of $34 million for these forward contracts in 2011 with an offsetting obligation recorded in other long-term liabilities and deferred credits. Alliant Energy and IPL concluded that $30 million of the charges are probable of recovery from IPL’s customers, and therefore, were recorded to regulatory assets in 2011. The remaining $4 million of charges were determined not to be probable of recovery from IPL’s customers resulting in $2 million of charges related to electric customers recorded to “Electric production fuel and energy purchases” and $2 million of charges related to steam customers recorded to “Utility - Other operation and maintenance” in Alliant Energy’s and IPL’s Consolidated Statements of Income in 2011. In 2012, the D.C. Circuit Court vacated and remanded CSAPR for further revision to the EPA. The D.C. Circuit Court order also requires the EPA to continue administering CAIR pending the promulgation of a valid replacement for CSAPR. Despite CSAPR being vacated, the current value of these allowances continues to be nominal and significantly below the $34 million contract price for these allowances. Alliant Energy and IPL currently believe that CAIR will be replaced in the future, either by a modified CSAPR or another rule that addresses the interstate transport of air pollutants.Environmental-related costs - The IUB has permitted IPL to recover prudently incurred costs by allowing a representative level of MGP costs in the recoverable cost of service component of rates, as determined in its most recent retail gas rate case. Under the current rate-making treatment approved by the PSCW, the MGP expenditures of WPL are deferred and collected from retail gas customers over a five-year period after new rates are implemented. The MPUC allows the deferral of MGP-related costs applicable to IPL’s Minnesota sites and IPL has received approval to recover such costs in retail gas rates in Minnesota in its most recent retail gas rate case. Regulatory assets recorded by IPL and WPL reflect the probable future rate recovery of MGP expenditures. Refer to Note 16(e) for additional details of environmental-related MGP costs.Derivatives - In accordance with IPL’s and WPL’s fuel and natural gas recovery mechanisms, prudently incurred costs from derivative instruments are recovered from customers in the future after any losses are realized and gains from derivative instruments are refunded to customers in the future after any gains are realized. Based on these recovery mechanisms, the changes in the fair value of derivative liabilities/assets resulted in comparable changes to regulatory assets/liabilities on the Consolidated Balance Sheets. Refer to Note 15 for additional details of derivative assets and derivative liabilities.Debt redemption costs - For debt retired early with no subsequent re-issuance, IPL and WPL defer any debt repayment premiums and unamortized debt issuance costs and discounts as regulatory assets. These regulatory assets are amortized over the remaining original life of the debt retired early. Debt repayment premiums and other losses resulting from the refinancing of debt by IPL and WPL are deferred as regulatory assets and amortized over the life of the new debt issued.IPL’s electric transmission service costs - In 2010, IPL incurred electric transmission service costs billed by ITC under the Attachment “O” rate for ITC’s under-recovered 2008 costs. In 2010, the IUB issued an order authorizing IPL to defer the Iowa retail portion of these under-recovered costs and amortize the deferred costs over a five-year period ending December 2014. In accordance with this order, IPL is amortizing $8 million of this regulatory asset annually, with an equal and offsetting amount of amortization of IPL’s regulatory liability related to its electric transmission assets sale. The IUB determined that IPL should not include the unamortized balance of these deferred costs in electric rate base during the five-year recovery period.Other - Alliant Energy, IPL and WPL assess whether IPL’s and WPL’s regulatory assets are probable of future recovery by considering factors such as applicable regulations, recent orders by the applicable regulatory agencies, historical treatment of similar costs by the applicable regulatory agencies and regulatory environment changes. Based on these assessments, Alliant Energy, IPL and WPL believe the regulatory assets recognized as of December 31, 2013 in the above table are probable of future recovery. However, no assurance can be made that IPL and WPL will recover all of these regulatory assets in future rates. If future recovery of a regulatory asset ceases to be probable, the regulatory asset will be charged to expense in the period in which future recovery ceases to be probable.Based on the PSCW’s July 2012 order related to WPL’s 2013/2014 test period Wisconsin retail electric and gas rate case, WPL was authorized to recover previously incurred costs associated with the acquisition of a 25% ownership interest in Edgewater Unit 5 and proposed emission controls projects. As a result, Alliant Energy and WPL recorded a $5 million increase to regulatory assets, and a $5 million credit to “Utility - Other operation and maintenance” in their Consolidated Statements of Income in 2012.Based on assessments completed in 2011, Alliant Energy, IPL and WPL recognized impairment charges of $9 million, $2 million and $7 million, respectively, for regulatory assets that were no longer probable of future recovery. The regulatory asset impairment charges were recorded by Alliant Energy, IPL and WPL as reductions in regulatory assets and charges to “Utility - Other operation and maintenance” in their Consolidated Statements of Income in 2011.Based on the MPUC’s November 2011 order related to IPL’s 2009 test year Minnesota retail electric rate case, IPL was authorized to recover $2 million of previously incurred plant cancellation costs for Sutherland #4 over a 25-year period ending in 2037. As a result, Alliant Energy and IPL recorded a $2 million increase to regulatory assets, and a $2 million credit to “Utility - Other operation and maintenance” in their Consolidated Statements of Income in 2011.Regulatory Liabilities - At December 31, regulatory liabilities were comprised of the following items (in millions): Alliant Energy IPL WPL 2013 2012 2013 2012 2013 2012Cost of removal obligations$418.9 $408.7 $277.7 $268.0 $141.2 $140.7IPL’s tax benefit riders265.4 355.8 265.4 355.8 — —Energy conservation cost recovery52.7 55.1 9.3 10.0 43.4 45.1IPL’s electric transmission assets sale21.6 32.5 21.6 32.5 — —IPL’s electric transmission cost recovery14.6 — 14.6 — — —Commodity cost recovery7.5 17.7 5.5 5.2 2.0 12.5Other40.8 46.3 20.8 29.9 20.0 16.4 $821.5 $916.1 $614.9 $701.4 $206.6 $214.7Regulatory liabilities related to cost of removal obligations, to the extent expensed through depreciation rates, reduce rate base. A significant portion of the remaining regulatory liabilities are not used to reduce rate base in the revenue requirement calculations utilized in IPL’s and WPL’s respective rate proceedings.Cost of removal obligations - Alliant Energy, IPL and WPL collect in rates future removal costs for many assets that do not have associated legal AROs. Alliant Energy, IPL and WPL record a regulatory liability for the estimated amounts they have collected in rates for these future removal costs less amounts spent on removal activities.IPL’s tax benefit riders - At December 31, 2013, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities in the above table consisted of $228.9 million and $36.5 million for the electric and gas tax benefit riders, respectively. These regulatory liabilities decreased $90 million in aggregate in 2013 due to the following items:Electric tax benefit rider - In January 2011, the IUB approved an electric tax benefit rider proposed by IPL, which utilizes regulatory liabilities to credit bills of Iowa retail electric customers beginning in February 2011 to help offset the impact of rate increases on such customers. These regulatory liabilities are related to tax benefits from tax accounting method changes for repairs expenditures, allocation of mixed service costs and allocation of insurance proceeds from floods in 2008. Alliant Energy and IPL recognize an offsetting reduction to income tax expense for the after-tax amounts credited to IPL’s retail electric customers’ bills in Iowa, resulting in no impact to Alliant Energy’s and IPL’s net income from the electric tax benefit rider. In 2013, 2012 and 2011, Alliant Energy and IPL utilized $79 million, $83 million and $61 million, respectively, of electric tax benefit rider-related regulatory liabilities accumulated in prior years to credit IPL’s Iowa retail electric customers’ bills. In 2013, 2012 and 2011, the $79 million, $83 million and $61 million reductions to “Electric operating revenues” resulted in $33 million, $35 million and $25 million of credits to “Income tax expense (benefit)” as a result of the decrease in taxable income in Alliant Energy’s and IPL’s Consolidated Statements of Income in 2013, 2012 and 2011, respectively. In 2013, 2012 and 2011, additional reductions to “Income tax expense (benefit)” of $46 million, $48 million and $36 million, respectively, were also recognized in Alliant Energy’s and IPL’s Consolidated Statements of Income representing the tax benefits realized related to the electric tax benefit rider. In December 2013, the IUB issued an order authorizing $85 million of regulatory liabilities from tax benefits to be credited to IPL’s retail electric customers’ bills in Iowa during 2014 through the electric tax benefit rider.Gas tax benefit rider - In November 2012, the IUB approved a gas tax benefit rider proposed by IPL, which utilizes up to $12 million of regulatory liabilities annually to credit bills of Iowa retail gas customers beginning in January 2013 through December 2015 to help offset the impact of rate increases on such customers. These regulatory liabilities are related to tax benefits from tax accounting method changes for repairs expenditures, allocation of mixed service costs and allocation of insurance proceeds from floods in 2008. Any remaining benefit, including any portion not utilized of the agreed upon amount from January 2013 through December 2015, will be credited to Iowa’s retail gas customers’ bills in 2016. In 2013, Alliant Energy and IPL utilized $11 million of gas tax benefit rider-related regulatory liabilities to credit IPL’s Iowa retail gas customers’ bills. In 2013, the $11 million reduction to “Gas operating revenues” resulted in $4 million of credits to “Income tax expense (benefit)” as a result of the decrease in taxable income in Alliant Energy’s and IPL’s Consolidated Statements of Income in 2013. In 2013, additional reductions to “Income tax expense (benefit)” of $7 million were also recognized in Alliant Energy’s and IPL’s Consolidated Statements of Income representing the tax benefits realized related to the gas tax benefit rider.Refer to Note 11 for additional details regarding IPL’s tax benefit riders.Energy conservation cost recovery - WPL and IPL collect revenues from their customers to offset certain expenditures incurred by WPL and IPL for conservation programs, including state mandated programs and Shared Savings programs. Differences between forecasted costs used to set rates and actual costs for these programs are deferred as a regulatory asset or regulatory liability.IPL’s electric transmission assets sale - In 2007, IPL completed the sale of its electric transmission assets to ITC and recognized a gain based on the terms of the agreement. Upon closing of the sale, IPL established a regulatory liability of $89 million pursuant to conditions established by the IUB when it allowed the transaction to proceed. The regulatory liability represented the present value of IPL’s obligation to refund to its customers payments beginning in the year IPL’s customers experience an increase in rates related to the transmission charges assessed by ITC. The regulatory liability accrues interest at the monthly average U.S. Treasury rate for three-year maturities.Iowa retail portion - In 2009, the IUB issued an order authorizing IPL to use a portion of this regulatory liability to reduce Iowa retail electric customers’ rates by $12 million for the period from July 2009 through February 2010 with billing credits included in the monthly energy adjustment clause. In 2010, the IUB issued an order authorizing IPL to use a portion of this regulatory liability to offset electric transmission service costs expected to be billed to IPL by ITC in 2010 related to ITC’s 2008 transmission revenue adjustment. IPL expects to utilize $41 million of this regulatory liability over a five-year period ending December 2014 to offset the Iowa retail portion of transmission costs billed to IPL by ITC in 2010 related to ITC’s 2008 transmission revenue adjustment. As a result, IPL is amortizing $8 million of this regulatory liability annually, with an equal and offsetting amount of amortization for IPL’s regulatory asset related to electric transmission service costs.In accordance with the IUB’s 2011 order related to IPL’s 2009 test year Iowa retail electric rate case, IPL was authorized to utilize regulatory liabilities in 2011 to offset transmission service expenses related to the Iowa retail portion of 2009 under-recovered costs billed to IPL. As a result, Alliant Energy and IPL recorded a reduction of $19 million in regulatory liabilities, and a reduction of $19 million in “Electric transmission service” in their Consolidated Statements of Income in 2011. The IUB also authorized IPL to utilize $3 million of this regulatory liability in 2011 to reduce IPL’s Iowa retail electric rate base associated with the Whispering Willow - East wind project.Minnesota retail portion - In 2010, the MPUC issued an interim rate order authorizing IPL to use a portion of this regulatory liability to implement an alternative transaction adjustment through its energy adjustment clause resulting in annual credits to its Minnesota retail electric customers beginning in July 2010 to coincide with the effective date of the interim rate increase for Minnesota retail customers. The amounts of the annual credits are dependent upon the level of KWhs sold to IPL’s Minnesota retail customers. In accordance with the MPUC’s November 2011 order related to IPL’s 2009 test year Minnesota retail electric rate case, IPL was authorized to refund a higher amount of the gain realized from the sale of its electric transmission assets in 2007 to its Minnesota retail electric customers than previously estimated. As a result, Alliant Energy and IPL recorded a $5 million increase to regulatory liabilities, and a $5 million charge to “Utility - Other operation and maintenance” in their Consolidated Statements of Income in 2011 for the additional amount to be refunded.Refunds related to any remaining balance of IPL’s electric transmission assets sale regulatory liability are expected to be determined in future rate proceedings or as part of the proposed sale of Minnesota electric distribution assets.IPL’s electric transmission cost recovery - Refer to Note 1(g) for additional details of IPL’s electric transmission service cost recovery mechanism.Commodity cost recovery - Refer to Note 1(g) for additional details of IPL’s and WPL’s cost recovery mechanisms. Refer to “WPL’s Retail Fuel-related Rate Filing (2012 Test Year)” below for discussion of refunds made to WPL’s retail electric customers in 2013.Utility Rate Cases -WPL’s Wisconsin Retail Electric and Gas Rate Case (2013/2014 Test Period) - In July 2012, WPL received an order from the PSCW authorizing WPL to implement a decrease in annual base rates for WPL’s retail gas customers of $13 million effective January 1, 2013, followed by a freeze of such gas base rates through the end of 2014. The order also authorized WPL to maintain customer base rates for its retail electric customers at their current levels through the end of 2014. The order included provisions that require WPL to defer a portion of its earnings if its annual regulatory return on common equity exceeds certain levels during 2013 or 2014 and allows WPL to request a change in retail base rates during this period if its annual regulatory return on common equity falls below a certain level. As of December 31, 2013, Alliant Energy and WPL did not record any material deferred amounts for these provisions. Refer to “Regulatory Assets” above for discussion of regulatory-related credits recorded in 2012 as a result of the PSCW’s order authorizing WPL to recover previously incurred costs associated with the acquisition of a 25% interest in Edgewater Unit 5 and proposed emission controls projects.IPL’s Iowa Retail Gas Rate Case (2011 Test Year) - In May 2012, IPL filed a request with the IUB to increase annual rates for its Iowa retail gas customers based on a 2011 historical test year as adjusted for certain known and measurable changes occurring up to 12 months after the commencement of the proceeding. IPL’s request included a proposal to utilize regulatory liabilities to credit bills of Iowa retail gas customers to help mitigate the impact of the proposed final rate increase on such customers. IPL proposed to reduce customer bills utilizing a gas tax benefit rider over a three-year period by approximately $36 million in aggregate. In conjunction with the filing, IPL implemented an interim retail gas rate increase of $9 million, or approximately 3%, on an annual basis, effective June 4, 2012, without regulatory review and subject to refund pending determination of final rates from the request. In 2012, Alliant Energy and IPL recorded $5 million in gas revenues from IPL’s Iowa retail gas customers related to the interim retail gas rate increase. In November 2012, the IUB approved a settlement agreement between IPL, the OCA and the Iowa Consumers Coalition related to IPL’s request, resulting in a final increase in annual rates for IPL’s Iowa retail gas customers of $11 million, or approximately 4%, effective January 10, 2013. The parties and the IUB also agreed to IPL’s proposed gas tax benefit rider. In 2013, Alliant Energy and IPL recorded $11 million in gas revenues from IPL’s Iowa retail gas customers related to the final retail gas rate increase. Refer to “Regulatory Liabilities” above for additional details on IPL’s gas tax benefit rider.IPL’s Iowa Retail Electric Rate Case (2009 Test Year) - Electric Tax Benefit Rider - In February 2013, the IUB issued an order allowing IPL to recognize a revenue requirement adjustment of $24 million in 2013 related to certain tax benefits from tax accounting method changes. The revenue requirement adjustment is recognized through the energy adjustment clause as a reduction of the credits on IPL’s Iowa retail electric customers’ bills from the electric tax benefit rider. In 2013, Alliant Energy and IPL recognized $24 million of the revenue requirement adjustment resulting in increases to electric revenues in their Consolidated Statements of Income. In December 2013, the IUB authorized IPL to reduce the billing credits on customers’ bills by $15 million in 2014 from tax benefits for the electric tax benefit rider to recognize the revenue requirement impact of the changes in tax accounting methods.IPL’s Minnesota Retail Electric Rate Case (2009 Test Year) - In May 2010, IPL filed a request with the MPUC to increase annual rates for its Minnesota retail electric customers based on a 2009 historical test year as adjusted for certain known and measurable items at the time of the filing. The key drivers for the filing included recovery of investments in the Whispering Willow - East wind project and emission controls projects at Lansing Unit 4, and recovery of increased electric transmission service costs. In conjunction with the filing, IPL implemented an interim retail rate increase of $14 million, on an annual basis, effective July 6, 2010. In November 2011, IPL received an order from the MPUC authorizing a final annual retail electric rate increase equivalent to $11 million. The final annual retail electric rate increase of $11 million includes $8 million of higher base rates, $2 million from the temporary renewable energy rider and $1 million from the utilization of regulatory liabilities to offset higher electric transmission service costs. Refer to “Regulatory Assets” and “Regulatory Liabilities” above for discussion of changes to regulatory assets and regulatory liabilities in 2011 based on the MPUC’s decisions in this rate case. Refer to Note 3(a) for discussion of adjustments made by Alliant Energy and IPL in 2011 and 2013 to the carrying value of IPL’s Whispering Willow - East wind project, based on amounts IPL determined were probable of being disallowed for recovery from its Minnesota retail electric customers.WPL’s Retail Fuel-related Rate Filing (2014 Test Year) - In December 2013, WPL received an order from the PSCW authorizing an annual retail electric rate increase of $19 million, or approximately 2%, effective January 1, 2014 to reflect anticipated increases in retail fuel-related costs in 2014 compared to the fuel-related cost estimates used to determine rates for 2013. WPL’s 2014 fuel-related costs will be subject to deferral if they fall outside an annual bandwidth of plus or minus 2% of the approved annual forecasted fuel-related costs.WPL’s Retail Fuel-related Rate Filing (2013 Test Year) - In December 2012, WPL received an order from the PSCW authorizing an annual retail electric rate decrease of $29 million, or approximately 3%, effective January 1, 2013 to reflect anticipated decreases in retail fuel-related costs in 2013 compared to the fuel-related cost estimates used to determine rates for 2012. WPL’s 2013 fuel-related costs were subject to deferral if they fell outside an annual bandwidth of plus or minus 2% of the approved annual forecasted fuel-related costs. Retail fuel-related costs incurred by WPL for 2013 did not fall outside of the fuel monitoring range.WPL’s Retail Fuel-related Rate Filing (2012 Test Year) - In December 2011, WPL received an order from the PSCW authorizing an annual retail electric rate increase of $4 million, effective January 1, 2012 to reflect anticipated increases in retail fuel-related costs in 2012 compared to the fuel-related cost estimates used to determine rates for 2011. The 2012 fuel-related costs were subject to an annual bandwidth of plus or minus 2%. Retail fuel-related costs incurred by WPL in 2012 were lower than retail fuel-related costs used to determine rates for such period resulting in an over-collection of fuel-related costs for 2012 of approximately $17 million (including $11 million outside the approved range for 2012 recorded in “Regulatory liabilities” on Alliant Energy’s and WPL’s Consolidated Balance Sheets as of December 31, 2012). In August 2013, WPL received an order from the PSCW to refund $12 million, including interest, to its retail electric customers for these over-collections, which WPL completed in September 2013.Refer to Note 1(g) for further discussion of WPL’s fuel cost recovery mechanism.