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Regulatory Matters | 12 Months Ended
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Dec. 31, 2015
Regulatory Matters [Line Items] |
Regulatory Matters | REGULATORY MATTERS Regulatory Assets - At December 31, regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014
Tax-related |  | $987.7 |  |  |  | $955.3 |  |  |  | $958.2 |  |  |  | $928.0 |  |  |  | $29.5 |  |  |  | $27.3 |
Pension and OPEB costs | 579.5 |  |  | 570.2 |  |  | 298.1 |  |  | 287.9 |  |  | 281.4 |  |  | 282.3 |
AROs | 92.4 |  |  | 73.7 |  |  | 50.8 |  |  | 41.4 |  |  | 41.6 |  |  | 32.3 |
Derivatives | 70.6 |  |  | 46.9 |  |  | 28.2 |  |  | 28.0 |  |  | 42.4 |  |  | 18.9 |
WPL’s EGUs retired early | 45.0 |  |  | — |  |  | — |  |  | — |  |  | 45.0 |  |  | — |
Commodity cost recovery | 35.9 |  |  | 31.1 |  |  | 2.8 |  |  | 0.4 |  |  | 33.1 |  |  | 30.7 |
Emission allowances | 26.9 |  |  | 27.4 |  |  | 26.9 |  |  | 27.4 |  |  | — |  |  | — |
Environmental-related costs | 16.4 |  |  | 16.0 |  |  | 11.0 |  |  | 11.5 |  |  | 5.4 |  |  | 4.5 |
Debt redemption costs | 14.3 |  |  | 16.1 |  |  | 9.5 |  |  | 10.9 |  |  | 4.8 |  |  | 5.2 |
Other | 39.9 |  |  | 47.0 |  |  | 17.1 |  |  | 22.4 |  |  | 22.8 |  |  | 24.6 |
|  | $1,908.6 |  |  |  | $1,783.7 |  |  |  | $1,402.6 |  |  |  | $1,357.9 |  |  |  | $506.0 |  |  |  | $425.8 |
A 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, 2015, IPL and WPL had $30 million and $12 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 and clean air compliance projects costs, and emission allowances. WPL’s regulatory assets that were not earning a return consisted primarily of amounts related to the wholesale portion of under-collected fuel-related costs, which is discussed in Note 1(g), and environmental-related costs. 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 2015, Alliant Energy’s and IPL’s tax-related regulatory assets increased primarily due to property-related differences for qualifying repair expenditures, partially offset by a reduction in certain tax-related regulatory assets due to the sales of IPL’s Minnesota electric and natural gas distribution assets in 2015. 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 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 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. Pension and OPEB 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 OPEB costs determined in accordance with GAAP and are calculated using different methods for the various regulatory jurisdictions in which IPL and WPL operate. The IUB authorized IPL in its 2009 Test Year 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 2009 Test Year and an estimate of costs for its forward-looking post-Test Year (2010). The PSCW authorized WPL in its Wisconsin retail rate cases for the 2013/2014 and 2015/2016 Test Periods to recover from its electric and gas retail customers an estimated allocated portion of annual costs equal to the costs expected to be incurred during the respective test year periods. IPL and WPL are authorized to recover from their 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 pension costs each year are refunded to or recovered from customers through subsequent changes to wholesale customer rates. Refer to Note 12(a) for additional details regarding pension and OPEB 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. Derivatives - In accordance with IPL’s and WPL’s fuel and natural gas recovery mechanisms, prudently incurred costs from derivative instruments are recoverable from customers in the future after any losses are realized and gains from derivative instruments are refundable 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 balance sheets. Refer to Note 15 for additional details of derivative assets and derivative liabilities. WPL’s electric generating units retired early - In December 2015, WPL retired Nelson Dewey Units 1 and 2 and Edgewater Unit 3. WPL received approval from FERC and the PSCW to reclassify the remaining net book value of these EGUs from property, plant and equipment to a regulatory asset on Alliant Energy’s and WPL’s balance sheets. The remaining net book value is included in WPL’s rate base and WPL is earning a return on the outstanding balance. WPL is currently recovering the remaining net book value of these EGUs from both its retail and wholesale customers over a 10-year period beginning January 1, 2013 pursuant to orders previously received by the PSCW and FERC. Continued recovery of the retail portion of the remaining net book value of these EGUs is expected to be addressed in WPL’s next retail electric base rate case. Commodity cost recovery - Refer to Note 1(g) for additional details of IPL’s and WPL’s cost recovery mechanisms. Emission allowances - IPL entered into forward contracts in 2007 to purchase SO2 emission allowances with vintage years of 2014 through 2017 from various counterparties to meet future CAIR emission reduction standards. Any SO2 emission allowances acquired under these forward contracts could 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 contract price for these allowances. As a result, Alliant Energy and IPL recognized an obligation and offsetting regulatory assets for the amount of the forward contracts that are probable of recovery from IPL’s customers. The current value of these allowances continues to be nominal and significantly below the contract price for these allowances. 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. 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. 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. 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, 2015 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 the likelihood of future recovery is less than probable. Regulatory Liabilities - At December 31, regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014
Cost of removal obligations |  | $406.0 |  |  |  | $421.7 |  |  |  | $260.4 |  |  |  | $279.1 |  |  |  | $145.6 |  |  |  | $142.6 |
IPL’s tax benefit riders | 159.2 |  |  | 243.0 |  |  | 159.2 |  |  | 243.0 |  |  | — |  |  | — |
Energy efficiency cost recovery | 48.3 |  |  | 64.3 |  |  | — |  |  | — |  |  | 48.3 |  |  | 64.3 |
Electric transmission cost recovery | 43.5 |  |  | 19.4 |  |  | 21.9 |  |  | 19.4 |  |  | 21.6 |  |  | — |
Commodity cost recovery | 37.6 |  |  | 15.4 |  |  | 23.5 |  |  | 15.1 |  |  | 14.1 |  |  | 0.3 |
Other | 43.1 |  |  | 57.4 |  |  | 24.2 |  |  | 26.9 |  |  | 18.9 |  |  | 30.5 |
|  | $737.7 |  |  |  | $821.2 |  |  |  | $489.2 |  |  |  | $583.5 |  |  |  | $248.5 |  |  |  | $237.7 |
Regulatory 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 and reduce the regulatory liability for amounts spent on removal activities. IPL’s tax benefit riders - IPL’s tax benefit riders utilize regulatory liabilities to credit bills of IPL’s Iowa retail electric and gas customers 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, allocation of insurance proceeds from floods in 2008, and cost of removal expenditures. In 2015, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities decreased by $84 million as follows (in millions):
Electric tax benefit rider credits |  | $72 |
Gas tax benefit rider credits | 12 |
|  | $84 |
Electric tax benefit rider - The IUB has approved an electric tax benefit rider proposed by IPL, which utilizes regulatory liabilities to credit bills of Iowa retail electric customers beginning in 2011 to help offset the impact of rate increases on such customers. 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 as follows (in millions):
| 2015 |  | 2014 |  | 2013
Credit to IPL’s Iowa retail electric customers’ bills with reduction to electric revenues |  | $72 |  |  |  | $85 |  |  |  | $79 |
Income tax benefit resulting from decreased taxable income caused by credits | 30 |  |  | 35 |  |  | 33 |
Income tax benefit representing tax benefits realized from electric tax benefit rider | 42 |  |  | 50 |  |  | 46 |
In December 2015, the IUB issued an order authorizing $65 million of regulatory liabilities from tax benefits to be credited to IPL’s retail electric customers’ bills in Iowa in 2016 through the electric tax benefit rider. The IUB has authorized IPL to reduce the electric tax benefit rider billing credits on customers’ bills to recognize the revenue requirement impact of the changes in tax accounting methods related to tangible property and mixed service costs. The revenue requirement adjustment resulted in increases to electric revenues in Alliant Energy’s and IPL’s income statements and was 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 as follows (in millions):
| 2015 |  | 2014 |  | 2013
Revenue requirement adjustment |  | $14 |  |  |  | $15 |  |  |  | $24 |
In December 2015, the IUB authorized IPL to reduce the $65 million of billing credits on customers’ bills by $15 million in 2016 to recognize the revenue requirement impact of the changes in tax accounting methods. Gas tax benefit rider - The IUB has approved a gas tax benefit rider proposed by IPL, which utilizes approximately $12 million of regulatory liabilities annually to credit bills of Iowa retail gas customers beginning in January 2013 to help offset the impact of rate increases on such customers. Any remaining benefit, including any portion not utilized of the agreed upon amount from January 2013 through December 2015, is expected to be credited to Iowa’s retail gas customers’ bills in 2016. Alliant Energy and IPL utilized gas tax benefit rider-related regulatory liabilities to credit IPL’s Iowa retail gas customers’ bills as follows (in millions):
| 2015 |  | 2014 |  | 2013
Credit to IPL’s Iowa retail gas customers’ bills with reduction to gas revenues |  | $12 |  |  |  | $12 |  |  |  | $11 |
Income tax benefit resulting from decreased taxable income caused by credits | 5 |  |  | 5 |  |  | 4 |
Income tax benefit representing tax benefits realized from gas tax benefit rider | 7 |  |  | 7 |  |  | 7 |
Refer to Note 11 for additional details regarding IPL’s tax benefit riders. Energy efficiency cost recovery - WPL and IPL collect revenues from their customers to offset certain expenditures incurred by WPL and IPL for energy efficiency 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. The July 2014 PSCW order for WPL’s 2015/2016 Test Period electric and gas base rate case authorized lower energy efficiency cost recovery amortizations for 2015, which contributed to the decrease in Alliant Energy’s and WPL’s “Energy efficiency cost recovery” regulatory liabilities. Electric transmission cost recovery - Electric revenues established in WPL’s retail electric rate case (2015/2016 Test Period) included recovery of expected increases in electric transmission service expense largely due to SSR costs expected to be incurred. Due to a revision in MISO’s method to allocate SSR costs, WPL no longer expects to incur certain SSR costs. The difference between actual electric transmission service expense incurred and amounts collected from customers as electric revenues in 2015 are recorded as electric transmission service expense with an offsetting amount recorded to regulatory liabilities due to the escrow treatment authorized for WPL in its 2015/2016 Test Period retail electric rate case. Refer to Note 1(g) for additional details of IPL’s and WPL’s electric transmission service cost recovery mechanisms. Commodity cost recovery - Refer to Note 1(g) for additional details of IPL’s and WPL’s commodity cost recovery mechanisms. Utility Rate Cases - WPL’s Wisconsin Retail Electric and Gas Rate Case (2015/2016 Test Period) - In July 2014, WPL received an order from the PSCW authorizing WPL to maintain retail electric base rates at their current levels through the end of 2016. The retail electric base rate case included a return of and a return on costs for environmental controls projects at Columbia Units 1 and 2 and Edgewater Unit 5, generation performance and reliability improvements at Columbia Units 1 and 2, other ongoing capital expenditures, and an increase in electric transmission service expense. The additional revenue requirement for these cost increases was offset by the impact of changes in the amortization of regulatory liabilities associated with energy efficiency cost recoveries and increased sales volumes. The order also authorized WPL to implement a $5 million decrease in annual base rates for its retail gas customers effective January 1, 2015 followed by a freeze of such gas base rates through the end of 2016. 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 2015 or 2016 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, 2015, Alliant Energy and WPL did not record any deferred amounts for these provisions. 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 retail gas base rates 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 retail electric base rates at their then current levels through the end of 2014. The order included a provision that required WPL to defer a portion of its earnings if its annual regulatory return on common equity exceeded certain levels during 2013 or 2014. As of December 31, 2015, Alliant Energy and WPL deferred $8 million of WPL’s 2013 and 2014 earnings for these provisions, which is included in “Other” in Alliant Energy’s and WPL’s regulatory liabilities tables above. WPL currently expects the deferred amount will be refunded to its customers in a future rate case or other proceeding. IPL’s Iowa Retail Electric Rate Settlement Agreement - The IUB approved a settlement agreement in 2014 related to rates charged to IPL’s Iowa retail electric customers. The settlement agreement extends IPL’s Iowa retail electric base rates authorized in its 2009 Test Year rate case through 2016 and provides targeted retail electric customer billing credits of $105 million in aggregate. IPL recorded such billing credits in 2014 and 2015, and estimates billing credits in 2016, as follows (in millions):
| Actual |  | Estimated
| 2014 |  | 2015 |  | 2016
Billing credits to reduce retail electric customers’ bills |  | $72 |  |  |  | $24 |  |  |  | $9 |
WPL’s Retail Fuel-related Rate Filing (2016 Test Year) - In December 2015, WPL received an order from the PSCW authorizing an annual retail electric rate increase of $7 million, or approximately 1%, effective January 1, 2016. The increase reflects anticipated increases in retail electric fuel-related costs in 2016, including impacts of increased sales volumes approved in the retail base rate case for 2016. WPL’s 2016 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. Deferral of under-collections are reduced to the extent WPL’s actual return on common equity exceeds the most recently authorized return on common equity. WPL’s Retail Fuel-related Rate Filing (2015 Test Year) - In December 2014, WPL received an order from the PSCW authorizing an annual retail electric rate increase of $39 million, or approximately 4%, effective January 1, 2015. The increase included $39 million of anticipated increases in retail electric fuel-related costs in 2015 attributable to $25 million for higher retail electric fuel-related costs per MWh anticipated in 2015 and $14 million from the impact of increased sales volumes approved in the retail electric base rate case for 2015. WPL’s 2015 fuel-related costs were subject to deferral if they were outside an annual bandwidth of plus or minus 2% of the approved annual forecasted fuel-related costs. Retail fuel-related costs incurred by WPL through December 31, 2015 were lower than fuel-related costs used to determine rates for such period resulting in an over-collection of fuel-related costs. As of December 31, 2015, fuel-related costs outside of the approved range were $10 million and are included in “Commodity cost recovery” in Alliant Energy’s and WPL’s regulatory liabilities table above. WPL’s Retail Fuel-related Rate Filing (2014 Test Year) - Pursuant to a 2013 PSCW order, WPL’s 2014 fuel-related costs were subject to deferral since they were outside an annual bandwidth of plus or minus 2% of the approved annual forecasted fuel-related costs. Retail fuel-related costs incurred by WPL through December 31, 2014 were higher than fuel-related costs used to determine rates for such period resulting in an under-collection of fuel-related costs for 2014 of $33 million (including $28 million outside the approved range for 2014). As of December 31, 2015 and 2014, the $28 million of deferred fuel-related costs were included in “Commodity cost recovery” in Alliant Energy’s and WPL’s regulatory assets table above. In July 2015, WPL received an order from the PSCW authorizing an annual retail electric rate increase of $28 million, or approximately 2%, effective January 1, 2016 to recover the 2014 Test Year deferred fuel-related costs.
IPL [Member] |
Regulatory Matters [Line Items] |
Regulatory Matters | REGULATORY MATTERS Regulatory Assets - At December 31, regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014
Tax-related |  | $987.7 |  |  |  | $955.3 |  |  |  | $958.2 |  |  |  | $928.0 |  |  |  | $29.5 |  |  |  | $27.3 |
Pension and OPEB costs | 579.5 |  |  | 570.2 |  |  | 298.1 |  |  | 287.9 |  |  | 281.4 |  |  | 282.3 |
AROs | 92.4 |  |  | 73.7 |  |  | 50.8 |  |  | 41.4 |  |  | 41.6 |  |  | 32.3 |
Derivatives | 70.6 |  |  | 46.9 |  |  | 28.2 |  |  | 28.0 |  |  | 42.4 |  |  | 18.9 |
WPL’s EGUs retired early | 45.0 |  |  | — |  |  | — |  |  | — |  |  | 45.0 |  |  | — |
Commodity cost recovery | 35.9 |  |  | 31.1 |  |  | 2.8 |  |  | 0.4 |  |  | 33.1 |  |  | 30.7 |
Emission allowances | 26.9 |  |  | 27.4 |  |  | 26.9 |  |  | 27.4 |  |  | — |  |  | — |
Environmental-related costs | 16.4 |  |  | 16.0 |  |  | 11.0 |  |  | 11.5 |  |  | 5.4 |  |  | 4.5 |
Debt redemption costs | 14.3 |  |  | 16.1 |  |  | 9.5 |  |  | 10.9 |  |  | 4.8 |  |  | 5.2 |
Other | 39.9 |  |  | 47.0 |  |  | 17.1 |  |  | 22.4 |  |  | 22.8 |  |  | 24.6 |
|  | $1,908.6 |  |  |  | $1,783.7 |  |  |  | $1,402.6 |  |  |  | $1,357.9 |  |  |  | $506.0 |  |  |  | $425.8 |
A 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, 2015, IPL and WPL had $30 million and $12 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 and clean air compliance projects costs, and emission allowances. WPL’s regulatory assets that were not earning a return consisted primarily of amounts related to the wholesale portion of under-collected fuel-related costs, which is discussed in Note 1(g), and environmental-related costs. 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 2015, Alliant Energy’s and IPL’s tax-related regulatory assets increased primarily due to property-related differences for qualifying repair expenditures, partially offset by a reduction in certain tax-related regulatory assets due to the sales of IPL’s Minnesota electric and natural gas distribution assets in 2015. 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 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 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. Pension and OPEB 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 OPEB costs determined in accordance with GAAP and are calculated using different methods for the various regulatory jurisdictions in which IPL and WPL operate. The IUB authorized IPL in its 2009 Test Year 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 2009 Test Year and an estimate of costs for its forward-looking post-Test Year (2010). The PSCW authorized WPL in its Wisconsin retail rate cases for the 2013/2014 and 2015/2016 Test Periods to recover from its electric and gas retail customers an estimated allocated portion of annual costs equal to the costs expected to be incurred during the respective test year periods. IPL and WPL are authorized to recover from their 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 pension costs each year are refunded to or recovered from customers through subsequent changes to wholesale customer rates. Refer to Note 12(a) for additional details regarding pension and OPEB 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. Derivatives - In accordance with IPL’s and WPL’s fuel and natural gas recovery mechanisms, prudently incurred costs from derivative instruments are recoverable from customers in the future after any losses are realized and gains from derivative instruments are refundable 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 balance sheets. Refer to Note 15 for additional details of derivative assets and derivative liabilities. WPL’s electric generating units retired early - In December 2015, WPL retired Nelson Dewey Units 1 and 2 and Edgewater Unit 3. WPL received approval from FERC and the PSCW to reclassify the remaining net book value of these EGUs from property, plant and equipment to a regulatory asset on Alliant Energy’s and WPL’s balance sheets. The remaining net book value is included in WPL’s rate base and WPL is earning a return on the outstanding balance. WPL is currently recovering the remaining net book value of these EGUs from both its retail and wholesale customers over a 10-year period beginning January 1, 2013 pursuant to orders previously received by the PSCW and FERC. Continued recovery of the retail portion of the remaining net book value of these EGUs is expected to be addressed in WPL’s next retail electric base rate case. Commodity cost recovery - Refer to Note 1(g) for additional details of IPL’s and WPL’s cost recovery mechanisms. Emission allowances - IPL entered into forward contracts in 2007 to purchase SO2 emission allowances with vintage years of 2014 through 2017 from various counterparties to meet future CAIR emission reduction standards. Any SO2 emission allowances acquired under these forward contracts could 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 contract price for these allowances. As a result, Alliant Energy and IPL recognized an obligation and offsetting regulatory assets for the amount of the forward contracts that are probable of recovery from IPL’s customers. The current value of these allowances continues to be nominal and significantly below the contract price for these allowances. 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. 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. 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. 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, 2015 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 the likelihood of future recovery is less than probable. Regulatory Liabilities - At December 31, regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014
Cost of removal obligations |  | $406.0 |  |  |  | $421.7 |  |  |  | $260.4 |  |  |  | $279.1 |  |  |  | $145.6 |  |  |  | $142.6 |
IPL’s tax benefit riders | 159.2 |  |  | 243.0 |  |  | 159.2 |  |  | 243.0 |  |  | — |  |  | — |
Energy efficiency cost recovery | 48.3 |  |  | 64.3 |  |  | — |  |  | — |  |  | 48.3 |  |  | 64.3 |
Electric transmission cost recovery | 43.5 |  |  | 19.4 |  |  | 21.9 |  |  | 19.4 |  |  | 21.6 |  |  | — |
Commodity cost recovery | 37.6 |  |  | 15.4 |  |  | 23.5 |  |  | 15.1 |  |  | 14.1 |  |  | 0.3 |
Other | 43.1 |  |  | 57.4 |  |  | 24.2 |  |  | 26.9 |  |  | 18.9 |  |  | 30.5 |
|  | $737.7 |  |  |  | $821.2 |  |  |  | $489.2 |  |  |  | $583.5 |  |  |  | $248.5 |  |  |  | $237.7 |
Regulatory 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 and reduce the regulatory liability for amounts spent on removal activities. IPL’s tax benefit riders - IPL’s tax benefit riders utilize regulatory liabilities to credit bills of IPL’s Iowa retail electric and gas customers 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, allocation of insurance proceeds from floods in 2008, and cost of removal expenditures. In 2015, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities decreased by $84 million as follows (in millions):
Electric tax benefit rider credits |  | $72 |
Gas tax benefit rider credits | 12 |
|  | $84 |
Electric tax benefit rider - The IUB has approved an electric tax benefit rider proposed by IPL, which utilizes regulatory liabilities to credit bills of Iowa retail electric customers beginning in 2011 to help offset the impact of rate increases on such customers. 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 as follows (in millions):
| 2015 |  | 2014 |  | 2013
Credit to IPL’s Iowa retail electric customers’ bills with reduction to electric revenues |  | $72 |  |  |  | $85 |  |  |  | $79 |
Income tax benefit resulting from decreased taxable income caused by credits | 30 |  |  | 35 |  |  | 33 |
Income tax benefit representing tax benefits realized from electric tax benefit rider | 42 |  |  | 50 |  |  | 46 |
In December 2015, the IUB issued an order authorizing $65 million of regulatory liabilities from tax benefits to be credited to IPL’s retail electric customers’ bills in Iowa in 2016 through the electric tax benefit rider. The IUB has authorized IPL to reduce the electric tax benefit rider billing credits on customers’ bills to recognize the revenue requirement impact of the changes in tax accounting methods related to tangible property and mixed service costs. The revenue requirement adjustment resulted in increases to electric revenues in Alliant Energy’s and IPL’s income statements and was 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 as follows (in millions):
| 2015 |  | 2014 |  | 2013
Revenue requirement adjustment |  | $14 |  |  |  | $15 |  |  |  | $24 |
In December 2015, the IUB authorized IPL to reduce the $65 million of billing credits on customers’ bills by $15 million in 2016 to recognize the revenue requirement impact of the changes in tax accounting methods. Gas tax benefit rider - The IUB has approved a gas tax benefit rider proposed by IPL, which utilizes approximately $12 million of regulatory liabilities annually to credit bills of Iowa retail gas customers beginning in January 2013 to help offset the impact of rate increases on such customers. Any remaining benefit, including any portion not utilized of the agreed upon amount from January 2013 through December 2015, is expected to be credited to Iowa’s retail gas customers’ bills in 2016. Alliant Energy and IPL utilized gas tax benefit rider-related regulatory liabilities to credit IPL’s Iowa retail gas customers’ bills as follows (in millions):
| 2015 |  | 2014 |  | 2013
Credit to IPL’s Iowa retail gas customers’ bills with reduction to gas revenues |  | $12 |  |  |  | $12 |  |  |  | $11 |
Income tax benefit resulting from decreased taxable income caused by credits | 5 |  |  | 5 |  |  | 4 |
Income tax benefit representing tax benefits realized from gas tax benefit rider | 7 |  |  | 7 |  |  | 7 |
Refer to Note 11 for additional details regarding IPL’s tax benefit riders. Energy efficiency cost recovery - WPL and IPL collect revenues from their customers to offset certain expenditures incurred by WPL and IPL for energy efficiency 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. The July 2014 PSCW order for WPL’s 2015/2016 Test Period electric and gas base rate case authorized lower energy efficiency cost recovery amortizations for 2015, which contributed to the decrease in Alliant Energy’s and WPL’s “Energy efficiency cost recovery” regulatory liabilities. Electric transmission cost recovery - Electric revenues established in WPL’s retail electric rate case (2015/2016 Test Period) included recovery of expected increases in electric transmission service expense largely due to SSR costs expected to be incurred. Due to a revision in MISO’s method to allocate SSR costs, WPL no longer expects to incur certain SSR costs. The difference between actual electric transmission service expense incurred and amounts collected from customers as electric revenues in 2015 are recorded as electric transmission service expense with an offsetting amount recorded to regulatory liabilities due to the escrow treatment authorized for WPL in its 2015/2016 Test Period retail electric rate case. Refer to Note 1(g) for additional details of IPL’s and WPL’s electric transmission service cost recovery mechanisms. Commodity cost recovery - Refer to Note 1(g) for additional details of IPL’s and WPL’s commodity cost recovery mechanisms. Utility Rate Cases - WPL’s Wisconsin Retail Electric and Gas Rate Case (2015/2016 Test Period) - In July 2014, WPL received an order from the PSCW authorizing WPL to maintain retail electric base rates at their current levels through the end of 2016. The retail electric base rate case included a return of and a return on costs for environmental controls projects at Columbia Units 1 and 2 and Edgewater Unit 5, generation performance and reliability improvements at Columbia Units 1 and 2, other ongoing capital expenditures, and an increase in electric transmission service expense. The additional revenue requirement for these cost increases was offset by the impact of changes in the amortization of regulatory liabilities associated with energy efficiency cost recoveries and increased sales volumes. The order also authorized WPL to implement a $5 million decrease in annual base rates for its retail gas customers effective January 1, 2015 followed by a freeze of such gas base rates through the end of 2016. 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 2015 or 2016 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, 2015, Alliant Energy and WPL did not record any deferred amounts for these provisions. 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 retail gas base rates 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 retail electric base rates at their then current levels through the end of 2014. The order included a provision that required WPL to defer a portion of its earnings if its annual regulatory return on common equity exceeded certain levels during 2013 or 2014. As of December 31, 2015, Alliant Energy and WPL deferred $8 million of WPL’s 2013 and 2014 earnings for these provisions, which is included in “Other” in Alliant Energy’s and WPL’s regulatory liabilities tables above. WPL currently expects the deferred amount will be refunded to its customers in a future rate case or other proceeding. IPL’s Iowa Retail Electric Rate Settlement Agreement - The IUB approved a settlement agreement in 2014 related to rates charged to IPL’s Iowa retail electric customers. The settlement agreement extends IPL’s Iowa retail electric base rates authorized in its 2009 Test Year rate case through 2016 and provides targeted retail electric customer billing credits of $105 million in aggregate. IPL recorded such billing credits in 2014 and 2015, and estimates billing credits in 2016, as follows (in millions):
| Actual |  | Estimated
| 2014 |  | 2015 |  | 2016
Billing credits to reduce retail electric customers’ bills |  | $72 |  |  |  | $24 |  |  |  | $9 |
WPL’s Retail Fuel-related Rate Filing (2016 Test Year) - In December 2015, WPL received an order from the PSCW authorizing an annual retail electric rate increase of $7 million, or approximately 1%, effective January 1, 2016. The increase reflects anticipated increases in retail electric fuel-related costs in 2016, including impacts of increased sales volumes approved in the retail base rate case for 2016. WPL’s 2016 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. Deferral of under-collections are reduced to the extent WPL’s actual return on common equity exceeds the most recently authorized return on common equity. WPL’s Retail Fuel-related Rate Filing (2015 Test Year) - In December 2014, WPL received an order from the PSCW authorizing an annual retail electric rate increase of $39 million, or approximately 4%, effective January 1, 2015. The increase included $39 million of anticipated increases in retail electric fuel-related costs in 2015 attributable to $25 million for higher retail electric fuel-related costs per MWh anticipated in 2015 and $14 million from the impact of increased sales volumes approved in the retail electric base rate case for 2015. WPL’s 2015 fuel-related costs were subject to deferral if they were outside an annual bandwidth of plus or minus 2% of the approved annual forecasted fuel-related costs. Retail fuel-related costs incurred by WPL through December 31, 2015 were lower than fuel-related costs used to determine rates for such period resulting in an over-collection of fuel-related costs. As of December 31, 2015, fuel-related costs outside of the approved range were $10 million and are included in “Commodity cost recovery” in Alliant Energy’s and WPL’s regulatory liabilities table above. WPL’s Retail Fuel-related Rate Filing (2014 Test Year) - Pursuant to a 2013 PSCW order, WPL’s 2014 fuel-related costs were subject to deferral since they were outside an annual bandwidth of plus or minus 2% of the approved annual forecasted fuel-related costs. Retail fuel-related costs incurred by WPL through December 31, 2014 were higher than fuel-related costs used to determine rates for such period resulting in an under-collection of fuel-related costs for 2014 of $33 million (including $28 million outside the approved range for 2014). As of December 31, 2015 and 2014, the $28 million of deferred fuel-related costs were included in “Commodity cost recovery” in Alliant Energy’s and WPL’s regulatory assets table above. In July 2015, WPL received an order from the PSCW authorizing an annual retail electric rate increase of $28 million, or approximately 2%, effective January 1, 2016 to recover the 2014 Test Year deferred fuel-related costs.
WPL [Member] |
Regulatory Matters [Line Items] |
Regulatory Matters | REGULATORY MATTERS Regulatory Assets - At December 31, regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014
Tax-related |  | $987.7 |  |  |  | $955.3 |  |  |  | $958.2 |  |  |  | $928.0 |  |  |  | $29.5 |  |  |  | $27.3 |
Pension and OPEB costs | 579.5 |  |  | 570.2 |  |  | 298.1 |  |  | 287.9 |  |  | 281.4 |  |  | 282.3 |
AROs | 92.4 |  |  | 73.7 |  |  | 50.8 |  |  | 41.4 |  |  | 41.6 |  |  | 32.3 |
Derivatives | 70.6 |  |  | 46.9 |  |  | 28.2 |  |  | 28.0 |  |  | 42.4 |  |  | 18.9 |
WPL’s EGUs retired early | 45.0 |  |  | — |  |  | — |  |  | — |  |  | 45.0 |  |  | — |
Commodity cost recovery | 35.9 |  |  | 31.1 |  |  | 2.8 |  |  | 0.4 |  |  | 33.1 |  |  | 30.7 |
Emission allowances | 26.9 |  |  | 27.4 |  |  | 26.9 |  |  | 27.4 |  |  | — |  |  | — |
Environmental-related costs | 16.4 |  |  | 16.0 |  |  | 11.0 |  |  | 11.5 |  |  | 5.4 |  |  | 4.5 |
Debt redemption costs | 14.3 |  |  | 16.1 |  |  | 9.5 |  |  | 10.9 |  |  | 4.8 |  |  | 5.2 |
Other | 39.9 |  |  | 47.0 |  |  | 17.1 |  |  | 22.4 |  |  | 22.8 |  |  | 24.6 |
|  | $1,908.6 |  |  |  | $1,783.7 |  |  |  | $1,402.6 |  |  |  | $1,357.9 |  |  |  | $506.0 |  |  |  | $425.8 |
A 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, 2015, IPL and WPL had $30 million and $12 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 and clean air compliance projects costs, and emission allowances. WPL’s regulatory assets that were not earning a return consisted primarily of amounts related to the wholesale portion of under-collected fuel-related costs, which is discussed in Note 1(g), and environmental-related costs. 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 2015, Alliant Energy’s and IPL’s tax-related regulatory assets increased primarily due to property-related differences for qualifying repair expenditures, partially offset by a reduction in certain tax-related regulatory assets due to the sales of IPL’s Minnesota electric and natural gas distribution assets in 2015. 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 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 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. Pension and OPEB 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 OPEB costs determined in accordance with GAAP and are calculated using different methods for the various regulatory jurisdictions in which IPL and WPL operate. The IUB authorized IPL in its 2009 Test Year 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 2009 Test Year and an estimate of costs for its forward-looking post-Test Year (2010). The PSCW authorized WPL in its Wisconsin retail rate cases for the 2013/2014 and 2015/2016 Test Periods to recover from its electric and gas retail customers an estimated allocated portion of annual costs equal to the costs expected to be incurred during the respective test year periods. IPL and WPL are authorized to recover from their 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 pension costs each year are refunded to or recovered from customers through subsequent changes to wholesale customer rates. Refer to Note 12(a) for additional details regarding pension and OPEB 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. Derivatives - In accordance with IPL’s and WPL’s fuel and natural gas recovery mechanisms, prudently incurred costs from derivative instruments are recoverable from customers in the future after any losses are realized and gains from derivative instruments are refundable 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 balance sheets. Refer to Note 15 for additional details of derivative assets and derivative liabilities. WPL’s electric generating units retired early - In December 2015, WPL retired Nelson Dewey Units 1 and 2 and Edgewater Unit 3. WPL received approval from FERC and the PSCW to reclassify the remaining net book value of these EGUs from property, plant and equipment to a regulatory asset on Alliant Energy’s and WPL’s balance sheets. The remaining net book value is included in WPL’s rate base and WPL is earning a return on the outstanding balance. WPL is currently recovering the remaining net book value of these EGUs from both its retail and wholesale customers over a 10-year period beginning January 1, 2013 pursuant to orders previously received by the PSCW and FERC. Continued recovery of the retail portion of the remaining net book value of these EGUs is expected to be addressed in WPL’s next retail electric base rate case. Commodity cost recovery - Refer to Note 1(g) for additional details of IPL’s and WPL’s cost recovery mechanisms. Emission allowances - IPL entered into forward contracts in 2007 to purchase SO2 emission allowances with vintage years of 2014 through 2017 from various counterparties to meet future CAIR emission reduction standards. Any SO2 emission allowances acquired under these forward contracts could 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 contract price for these allowances. As a result, Alliant Energy and IPL recognized an obligation and offsetting regulatory assets for the amount of the forward contracts that are probable of recovery from IPL’s customers. The current value of these allowances continues to be nominal and significantly below the contract price for these allowances. 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. 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. 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. 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, 2015 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 the likelihood of future recovery is less than probable. Regulatory Liabilities - At December 31, regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014
Cost of removal obligations |  | $406.0 |  |  |  | $421.7 |  |  |  | $260.4 |  |  |  | $279.1 |  |  |  | $145.6 |  |  |  | $142.6 |
IPL’s tax benefit riders | 159.2 |  |  | 243.0 |  |  | 159.2 |  |  | 243.0 |  |  | — |  |  | — |
Energy efficiency cost recovery | 48.3 |  |  | 64.3 |  |  | — |  |  | — |  |  | 48.3 |  |  | 64.3 |
Electric transmission cost recovery | 43.5 |  |  | 19.4 |  |  | 21.9 |  |  | 19.4 |  |  | 21.6 |  |  | — |
Commodity cost recovery | 37.6 |  |  | 15.4 |  |  | 23.5 |  |  | 15.1 |  |  | 14.1 |  |  | 0.3 |
Other | 43.1 |  |  | 57.4 |  |  | 24.2 |  |  | 26.9 |  |  | 18.9 |  |  | 30.5 |
|  | $737.7 |  |  |  | $821.2 |  |  |  | $489.2 |  |  |  | $583.5 |  |  |  | $248.5 |  |  |  | $237.7 |
Regulatory 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 and reduce the regulatory liability for amounts spent on removal activities. IPL’s tax benefit riders - IPL’s tax benefit riders utilize regulatory liabilities to credit bills of IPL’s Iowa retail electric and gas customers 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, allocation of insurance proceeds from floods in 2008, and cost of removal expenditures. In 2015, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities decreased by $84 million as follows (in millions):
Electric tax benefit rider credits |  | $72 |
Gas tax benefit rider credits | 12 |
|  | $84 |
Electric tax benefit rider - The IUB has approved an electric tax benefit rider proposed by IPL, which utilizes regulatory liabilities to credit bills of Iowa retail electric customers beginning in 2011 to help offset the impact of rate increases on such customers. 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 as follows (in millions):
| 2015 |  | 2014 |  | 2013
Credit to IPL’s Iowa retail electric customers’ bills with reduction to electric revenues |  | $72 |  |  |  | $85 |  |  |  | $79 |
Income tax benefit resulting from decreased taxable income caused by credits | 30 |  |  | 35 |  |  | 33 |
Income tax benefit representing tax benefits realized from electric tax benefit rider | 42 |  |  | 50 |  |  | 46 |
In December 2015, the IUB issued an order authorizing $65 million of regulatory liabilities from tax benefits to be credited to IPL’s retail electric customers’ bills in Iowa in 2016 through the electric tax benefit rider. The IUB has authorized IPL to reduce the electric tax benefit rider billing credits on customers’ bills to recognize the revenue requirement impact of the changes in tax accounting methods related to tangible property and mixed service costs. The revenue requirement adjustment resulted in increases to electric revenues in Alliant Energy’s and IPL’s income statements and was 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 as follows (in millions):
| 2015 |  | 2014 |  | 2013
Revenue requirement adjustment |  | $14 |  |  |  | $15 |  |  |  | $24 |
In December 2015, the IUB authorized IPL to reduce the $65 million of billing credits on customers’ bills by $15 million in 2016 to recognize the revenue requirement impact of the changes in tax accounting methods. Gas tax benefit rider - The IUB has approved a gas tax benefit rider proposed by IPL, which utilizes approximately $12 million of regulatory liabilities annually to credit bills of Iowa retail gas customers beginning in January 2013 to help offset the impact of rate increases on such customers. Any remaining benefit, including any portion not utilized of the agreed upon amount from January 2013 through December 2015, is expected to be credited to Iowa’s retail gas customers’ bills in 2016. Alliant Energy and IPL utilized gas tax benefit rider-related regulatory liabilities to credit IPL’s Iowa retail gas customers’ bills as follows (in millions):
| 2015 |  | 2014 |  | 2013
Credit to IPL’s Iowa retail gas customers’ bills with reduction to gas revenues |  | $12 |  |  |  | $12 |  |  |  | $11 |
Income tax benefit resulting from decreased taxable income caused by credits | 5 |  |  | 5 |  |  | 4 |
Income tax benefit representing tax benefits realized from gas tax benefit rider | 7 |  |  | 7 |  |  | 7 |
Refer to Note 11 for additional details regarding IPL’s tax benefit riders. Energy efficiency cost recovery - WPL and IPL collect revenues from their customers to offset certain expenditures incurred by WPL and IPL for energy efficiency 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. The July 2014 PSCW order for WPL’s 2015/2016 Test Period electric and gas base rate case authorized lower energy efficiency cost recovery amortizations for 2015, which contributed to the decrease in Alliant Energy’s and WPL’s “Energy efficiency cost recovery” regulatory liabilities. Electric transmission cost recovery - Electric revenues established in WPL’s retail electric rate case (2015/2016 Test Period) included recovery of expected increases in electric transmission service expense largely due to SSR costs expected to be incurred. Due to a revision in MISO’s method to allocate SSR costs, WPL no longer expects to incur certain SSR costs. The difference between actual electric transmission service expense incurred and amounts collected from customers as electric revenues in 2015 are recorded as electric transmission service expense with an offsetting amount recorded to regulatory liabilities due to the escrow treatment authorized for WPL in its 2015/2016 Test Period retail electric rate case. Refer to Note 1(g) for additional details of IPL’s and WPL’s electric transmission service cost recovery mechanisms. Commodity cost recovery - Refer to Note 1(g) for additional details of IPL’s and WPL’s commodity cost recovery mechanisms. Utility Rate Cases - WPL’s Wisconsin Retail Electric and Gas Rate Case (2015/2016 Test Period) - In July 2014, WPL received an order from the PSCW authorizing WPL to maintain retail electric base rates at their current levels through the end of 2016. The retail electric base rate case included a return of and a return on costs for environmental controls projects at Columbia Units 1 and 2 and Edgewater Unit 5, generation performance and reliability improvements at Columbia Units 1 and 2, other ongoing capital expenditures, and an increase in electric transmission service expense. The additional revenue requirement for these cost increases was offset by the impact of changes in the amortization of regulatory liabilities associated with energy efficiency cost recoveries and increased sales volumes. The order also authorized WPL to implement a $5 million decrease in annual base rates for its retail gas customers effective January 1, 2015 followed by a freeze of such gas base rates through the end of 2016. 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 2015 or 2016 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, 2015, Alliant Energy and WPL did not record any deferred amounts for these provisions. 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 retail gas base rates 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 retail electric base rates at their then current levels through the end of 2014. The order included a provision that required WPL to defer a portion of its earnings if its annual regulatory return on common equity exceeded certain levels during 2013 or 2014. As of December 31, 2015, Alliant Energy and WPL deferred $8 million of WPL’s 2013 and 2014 earnings for these provisions, which is included in “Other” in Alliant Energy’s and WPL’s regulatory liabilities tables above. WPL currently expects the deferred amount will be refunded to its customers in a future rate case or other proceeding. IPL’s Iowa Retail Electric Rate Settlement Agreement - The IUB approved a settlement agreement in 2014 related to rates charged to IPL’s Iowa retail electric customers. The settlement agreement extends IPL’s Iowa retail electric base rates authorized in its 2009 Test Year rate case through 2016 and provides targeted retail electric customer billing credits of $105 million in aggregate. IPL recorded such billing credits in 2014 and 2015, and estimates billing credits in 2016, as follows (in millions):
| Actual |  | Estimated
| 2014 |  | 2015 |  | 2016
Billing credits to reduce retail electric customers’ bills |  | $72 |  |  |  | $24 |  |  |  | $9 |
WPL’s Retail Fuel-related Rate Filing (2016 Test Year) - In December 2015, WPL received an order from the PSCW authorizing an annual retail electric rate increase of $7 million, or approximately 1%, effective January 1, 2016. The increase reflects anticipated increases in retail electric fuel-related costs in 2016, including impacts of increased sales volumes approved in the retail base rate case for 2016. WPL’s 2016 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. Deferral of under-collections are reduced to the extent WPL’s actual return on common equity exceeds the most recently authorized return on common equity. WPL’s Retail Fuel-related Rate Filing (2015 Test Year) - In December 2014, WPL received an order from the PSCW authorizing an annual retail electric rate increase of $39 million, or approximately 4%, effective January 1, 2015. The increase included $39 million of anticipated increases in retail electric fuel-related costs in 2015 attributable to $25 million for higher retail electric fuel-related costs per MWh anticipated in 2015 and $14 million from the impact of increased sales volumes approved in the retail electric base rate case for 2015. WPL’s 2015 fuel-related costs were subject to deferral if they were outside an annual bandwidth of plus or minus 2% of the approved annual forecasted fuel-related costs. Retail fuel-related costs incurred by WPL through December 31, 2015 were lower than fuel-related costs used to determine rates for such period resulting in an over-collection of fuel-related costs. As of December 31, 2015, fuel-related costs outside of the approved range were $10 million and are included in “Commodity cost recovery” in Alliant Energy’s and WPL’s regulatory liabilities table above. WPL’s Retail Fuel-related Rate Filing (2014 Test Year) - Pursuant to a 2013 PSCW order, WPL’s 2014 fuel-related costs were subject to deferral since they were outside an annual bandwidth of plus or minus 2% of the approved annual forecasted fuel-related costs. Retail fuel-related costs incurred by WPL through December 31, 2014 were higher than fuel-related costs used to determine rates for such period resulting in an under-collection of fuel-related costs for 2014 of $33 million (including $28 million outside the approved range for 2014). As of December 31, 2015 and 2014, the $28 million of deferred fuel-related costs were included in “Commodity cost recovery” in Alliant Energy’s and WPL’s regulatory assets table above. In July 2015, WPL received an order from the PSCW authorizing an annual retail electric rate increase of $28 million, or approximately 2%, effective January 1, 2016 to recover the 2014 Test Year deferred fuel-related costs.

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