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Regulatory Matters | 12 Months Ended
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Dec. 31, 2016
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
| 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015
Tax-related |  | $1,055.6 |  |  |  | $987.7 |  |  |  | $1,022.4 |  |  |  | $958.2 |  |  |  | $33.2 |  |  |  | $29.5 |
Pension and OPEB costs | 578.7 |  |  | 579.5 |  |  | 294.0 |  |  | 298.1 |  |  | 284.7 |  |  | 281.4 |
AROs | 105.9 |  |  | 92.4 |  |  | 64.3 |  |  | 50.8 |  |  | 41.6 |  |  | 41.6 |
WPL’s EGUs retired early | 41.4 |  |  | 45.0 |  |  | — |  |  | — |  |  | 41.4 |  |  | 45.0 |
Derivatives | 30.7 |  |  | 70.6 |  |  | 10.0 |  |  | 28.2 |  |  | 20.7 |  |  | 42.4 |
Emission allowances | 26.2 |  |  | 26.9 |  |  | 26.2 |  |  | 26.9 |  |  | — |  |  | — |
Commodity cost recovery | 6.0 |  |  | 35.9 |  |  | 0.3 |  |  | 2.8 |  |  | 5.7 |  |  | 33.1 |
Other | 70.6 |  |  | 70.6 |  |  | 41.6 |  |  | 37.6 |  |  | 29.0 |  |  | 33.0 |
|  | $1,915.1 |  |  |  | $1,908.6 |  |  |  | $1,458.8 |  |  |  | $1,402.6 |  |  |  | $456.3 |  |  |  | $506.0 |
A portion of the regulatory assets in the above table are not earning a return. These regulatory assets, but not the respective carrying costs of these regulatory assets, are expected to be recovered from customers in future rates. At December 31, 2016, IPL and WPL had $44 million and $9 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 emission allowances, debt redemption costs, and costs for clean air compliance and wind generation expansion projects. 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 for IPL include the impacts of qualifying deductions for repairs expenditures, 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 2016, Alliant Energy’s and IPL’s tax-related regulatory assets increased primarily due to property-related differences for qualifying repair expenditures. 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 amounts 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 IPL’s and WPL’s respective regulatory jurisdictions. 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 has authorized WPL to recover from its retail electric and gas customers an estimated allocated portion of annual costs equal to the costs expected to be incurred during each test period. IPL and WPL are authorized to recover from their wholesale customers an allocated portion of actual pension costs incurred each year through FERC-approved formula 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. 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 the PSCW and FERC 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 PSCW and FERC orders. 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. 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. Alliant Energy and IPL have recorded a regulatory asset for amounts paid under the forward contracts, as well as for the remaining obligation under the forward contracts as a result of concluding the amount is probable of recovery from IPL’s customers. Commodity cost recovery - Refer to Note 1(g) for additional details of IPL’s and WPL’s cost recovery mechanisms. 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, 2016 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
| 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015
Cost of removal obligations |  | $411.6 |  |  |  | $406.0 |  |  |  | $269.4 |  |  |  | $260.4 |  |  |  | $142.2 |  |  |  | $145.6 |
IPL’s tax benefit riders | 83.5 |  |  | 159.2 |  |  | 83.5 |  |  | 159.2 |  |  | — |  |  | — |
Electric transmission cost recovery | 72.0 |  |  | 43.5 |  |  | 35.7 |  |  | 21.9 |  |  | 36.3 |  |  | 21.6 |
Derivatives | 31.5 |  |  | 8.5 |  |  | 12.1 |  |  | 6.7 |  |  | 19.4 |  |  | 1.8 |
Commodity cost recovery | 30.8 |  |  | 37.6 |  |  | 17.8 |  |  | 23.5 |  |  | 13.0 |  |  | 14.1 |
Energy efficiency cost recovery | 20.5 |  |  | 48.3 |  |  | — |  |  | — |  |  | 20.5 |  |  | 48.3 |
Other | 31.1 |  |  | 34.6 |  |  | 12.3 |  |  | 17.5 |  |  | 18.8 |  |  | 17.1 |
|  | $681.0 |  |  |  | $737.7 |  |  |  | $430.8 |  |  |  | $489.2 |  |  |  | $250.2 |  |  |  | $248.5 |
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 - The IUB has approved electric and gas tax benefit riders proposed by IPL, which 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. Alliant Energy and IPL recognize an offsetting reduction to income tax expense for the after-tax amounts credited to such customers, resulting in no impact to their net income from the electric and gas tax benefit riders. The tax benefit riders 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 2016, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities decreased by $76 million as follows (in millions):
Electric tax benefit rider credits |  | $64 |
Gas tax benefit rider credits | 12 |
|  | $76 |
In December 2016, the IUB authorized $68 million and $7 million of regulatory liabilities from tax benefits to be credited to IPL’s retail electric and gas customers’ bills in 2017 through the electric and gas tax benefit riders, respectively. Any remaining tax benefit rider regulatory liabilities are currently expected to be credited to IPL’s retail electric and gas customers’ bills in the future, subject to final review by the IUB. Electric tax benefit rider - Details for IPL’s electric tax benefit rider are as follows (in millions):
| 2016 |  | 2015 |  | 2014
Credit to IPL’s Iowa retail electric customers’ bills with reduction to electric revenues (based on customers’ KWh usage) |  | $64 |  |  |  | $72 |  |  |  | $85 |
Income tax benefit resulting from decreased taxable income caused by credits | 27 |  |  | 30 |  |  | 35 |
Income tax benefit representing tax benefits realized from electric tax benefit rider | 37 |  |  | 42 |  |  | 50 |
The IUB authorized IPL to reduce the electric tax benefit rider billing credits on customers’ bills for 2013 through 2016 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):
| 2016 |  | 2015 |  | 2014
Revenue requirement adjustment |  | $14 |  |  |  | $14 |  |  |  | $15 |
Gas tax benefit rider - Details for IPL’s gas tax benefit rider are as follows (in millions):
| 2016 |  | 2015 |  | 2014
Credit to IPL’s Iowa retail gas customers’ bills with reduction to gas revenues (based on a fixed amount per day) |  | $12 |  |  |  | $12 |  |  |  | $12 |
Income tax benefit resulting from decreased taxable income caused by credits | 5 |  |  | 5 |  |  | 5 |
Income tax benefit representing tax benefits realized from gas tax benefit rider | 7 |  |  | 7 |  |  | 7 |
Electric transmission cost recovery - 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. Energy efficiency cost recovery - WPL and IPL collect revenues from their customers to offset certain expenditures they each incur 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 PSCW’s order for WPL’s 2015/2016 Test Period electric and gas base rate case authorized energy efficiency cost recovery amortizations for 2016, which contributed to the decrease in Alliant Energy’s and WPL’s “Energy efficiency cost recovery” regulatory liabilities. Utility Rate Cases - WPL’s Wisconsin Retail Electric and Gas Rate Case (2017/2018 Test Period) - In December 2016, WPL received an order from the PSCW authorizing WPL to implement increases in annual retail electric and gas base rates of $9 million and $9 million, respectively, effective January 1, 2017 followed by a freeze of such rates through the end of 2018. The key drivers for the electric and gas base rate increases are recovery of the costs for environmental controls projects at Edgewater and Columbia, and investments in electric and gas distribution systems, including expansion of natural gas pipeline infrastructure. The filing also included utilization of amounts that WPL previously over-recovered from its customers for energy efficiency cost recovery and electric transmission cost recovery, as well as amounts deferred under the return on common equity sharing mechanism for the 2013/2014 Test Period to reduce the requested base rate increases. 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 in 2017 or 2018. 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 then current levels through the end of 2016. The retail electric base rate case included a return of and return of costs for environmental controls projects, generation performance and reliability improvements, 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 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 required WPL to defer a portion of its earnings if its annual regulatory return on common equity exceeded certain levels in 2015 or 2016. As of December 31, 2016, Alliant Energy and WPL deferred $6 million of WPL’s 2016 earnings for these provisions, which is included in “Other” in Alliant Energy’s and WPL’s regulatory liabilities tables above. WPL’s Wisconsin Retail Electric and Gas Rate Case (2013/2014 Test Period) - The PSCW’s order for WPL’s 2013/2014 Test Period electric and gas base rate case included provisions 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, 2016, Alliant Energy and WPL deferred $6 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. These deferred earnings will be returned to customers as an offset to revenue requirements in 2017 and 2018 as authorized by the PSCW. 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 extended IPL’s Iowa retail electric base rates authorized in its 2009 Test Year rate case through 2016 and provided targeted retail electric customer billing credits of $105 million in aggregate. IPL recorded such billing credits as follows (in millions):
| 2016 |  | 2015 |  | 2014
Billing credits to reduce retail electric customers’ bills |  | $9 |  |  |  | $24 |  |  |  | $72 |
WPL’s Retail Fuel-related Rate Filings - The PSCW authorized annual retail electric rate increases for WPL in 2014, 2015 and 2016, resulting from anticipated increases in retail electric fuel-related costs during such periods. Retail fuel-related costs incurred by WPL in 2015 and 2016 were lower than fuel-related costs used to determine rates for such period resulting in an over-collection of fuel-related costs, and retail fuel-related costs incurred by WPL in 2014 were higher than fuel-related costs used to determine rates for such period resulting in an under-collection of fuel-related costs. These fuel-related costs were subject to deferral since they were outside an annual bandwidth of plus or minus 2% of the approved respective annual forecasted fuel-related costs. Details on these rate increases, as well as amounts WPL deferred for the over-collected (included in “Commodity cost recovery” regulatory liabilities) or under-collected (included in “Commodity cost recovery” regulatory assets) fuel-related costs from its retail electric customers are as follows (dollars in millions):
|  | Retail Electric |  | Deferral of Over (Under) |  | Timing of Refunds To or
Year |  | Rate Increase |  | Collected Fuel-related Costs |  | Collections From Customers
2016 |  |  | $7 |  |  | 1% |  |  | $9 |  |  | Pending
2015 |  | 39 |  |  | 4% |  | 10 |  |  | 2016
2014 |  | 19 |  |  | 2% |  | (28 | ) |  | 2016
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
| 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015
Tax-related |  | $1,055.6 |  |  |  | $987.7 |  |  |  | $1,022.4 |  |  |  | $958.2 |  |  |  | $33.2 |  |  |  | $29.5 |
Pension and OPEB costs | 578.7 |  |  | 579.5 |  |  | 294.0 |  |  | 298.1 |  |  | 284.7 |  |  | 281.4 |
AROs | 105.9 |  |  | 92.4 |  |  | 64.3 |  |  | 50.8 |  |  | 41.6 |  |  | 41.6 |
WPL’s EGUs retired early | 41.4 |  |  | 45.0 |  |  | — |  |  | — |  |  | 41.4 |  |  | 45.0 |
Derivatives | 30.7 |  |  | 70.6 |  |  | 10.0 |  |  | 28.2 |  |  | 20.7 |  |  | 42.4 |
Emission allowances | 26.2 |  |  | 26.9 |  |  | 26.2 |  |  | 26.9 |  |  | — |  |  | — |
Commodity cost recovery | 6.0 |  |  | 35.9 |  |  | 0.3 |  |  | 2.8 |  |  | 5.7 |  |  | 33.1 |
Other | 70.6 |  |  | 70.6 |  |  | 41.6 |  |  | 37.6 |  |  | 29.0 |  |  | 33.0 |
|  | $1,915.1 |  |  |  | $1,908.6 |  |  |  | $1,458.8 |  |  |  | $1,402.6 |  |  |  | $456.3 |  |  |  | $506.0 |
A portion of the regulatory assets in the above table are not earning a return. These regulatory assets, but not the respective carrying costs of these regulatory assets, are expected to be recovered from customers in future rates. At December 31, 2016, IPL and WPL had $44 million and $9 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 emission allowances, debt redemption costs, and costs for clean air compliance and wind generation expansion projects. 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 for IPL include the impacts of qualifying deductions for repairs expenditures, 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 2016, Alliant Energy’s and IPL’s tax-related regulatory assets increased primarily due to property-related differences for qualifying repair expenditures. 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 amounts 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 IPL’s and WPL’s respective regulatory jurisdictions. 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 has authorized WPL to recover from its retail electric and gas customers an estimated allocated portion of annual costs equal to the costs expected to be incurred during each test period. IPL and WPL are authorized to recover from their wholesale customers an allocated portion of actual pension costs incurred each year through FERC-approved formula 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. 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 the PSCW and FERC 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 PSCW and FERC orders. 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. 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. Alliant Energy and IPL have recorded a regulatory asset for amounts paid under the forward contracts, as well as for the remaining obligation under the forward contracts as a result of concluding the amount is probable of recovery from IPL’s customers. Commodity cost recovery - Refer to Note 1(g) for additional details of IPL’s and WPL’s cost recovery mechanisms. 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, 2016 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
| 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015
Cost of removal obligations |  | $411.6 |  |  |  | $406.0 |  |  |  | $269.4 |  |  |  | $260.4 |  |  |  | $142.2 |  |  |  | $145.6 |
IPL’s tax benefit riders | 83.5 |  |  | 159.2 |  |  | 83.5 |  |  | 159.2 |  |  | — |  |  | — |
Electric transmission cost recovery | 72.0 |  |  | 43.5 |  |  | 35.7 |  |  | 21.9 |  |  | 36.3 |  |  | 21.6 |
Derivatives | 31.5 |  |  | 8.5 |  |  | 12.1 |  |  | 6.7 |  |  | 19.4 |  |  | 1.8 |
Commodity cost recovery | 30.8 |  |  | 37.6 |  |  | 17.8 |  |  | 23.5 |  |  | 13.0 |  |  | 14.1 |
Energy efficiency cost recovery | 20.5 |  |  | 48.3 |  |  | — |  |  | — |  |  | 20.5 |  |  | 48.3 |
Other | 31.1 |  |  | 34.6 |  |  | 12.3 |  |  | 17.5 |  |  | 18.8 |  |  | 17.1 |
|  | $681.0 |  |  |  | $737.7 |  |  |  | $430.8 |  |  |  | $489.2 |  |  |  | $250.2 |  |  |  | $248.5 |
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 - The IUB has approved electric and gas tax benefit riders proposed by IPL, which 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. Alliant Energy and IPL recognize an offsetting reduction to income tax expense for the after-tax amounts credited to such customers, resulting in no impact to their net income from the electric and gas tax benefit riders. The tax benefit riders 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 2016, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities decreased by $76 million as follows (in millions):
Electric tax benefit rider credits |  | $64 |
Gas tax benefit rider credits | 12 |
|  | $76 |
In December 2016, the IUB authorized $68 million and $7 million of regulatory liabilities from tax benefits to be credited to IPL’s retail electric and gas customers’ bills in 2017 through the electric and gas tax benefit riders, respectively. Any remaining tax benefit rider regulatory liabilities are currently expected to be credited to IPL’s retail electric and gas customers’ bills in the future, subject to final review by the IUB. Electric tax benefit rider - Details for IPL’s electric tax benefit rider are as follows (in millions):
| 2016 |  | 2015 |  | 2014
Credit to IPL’s Iowa retail electric customers’ bills with reduction to electric revenues (based on customers’ KWh usage) |  | $64 |  |  |  | $72 |  |  |  | $85 |
Income tax benefit resulting from decreased taxable income caused by credits | 27 |  |  | 30 |  |  | 35 |
Income tax benefit representing tax benefits realized from electric tax benefit rider | 37 |  |  | 42 |  |  | 50 |
The IUB authorized IPL to reduce the electric tax benefit rider billing credits on customers’ bills for 2013 through 2016 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):
| 2016 |  | 2015 |  | 2014
Revenue requirement adjustment |  | $14 |  |  |  | $14 |  |  |  | $15 |
Gas tax benefit rider - Details for IPL’s gas tax benefit rider are as follows (in millions):
| 2016 |  | 2015 |  | 2014
Credit to IPL’s Iowa retail gas customers’ bills with reduction to gas revenues (based on a fixed amount per day) |  | $12 |  |  |  | $12 |  |  |  | $12 |
Income tax benefit resulting from decreased taxable income caused by credits | 5 |  |  | 5 |  |  | 5 |
Income tax benefit representing tax benefits realized from gas tax benefit rider | 7 |  |  | 7 |  |  | 7 |
Electric transmission cost recovery - 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. Energy efficiency cost recovery - WPL and IPL collect revenues from their customers to offset certain expenditures they each incur 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 PSCW’s order for WPL’s 2015/2016 Test Period electric and gas base rate case authorized energy efficiency cost recovery amortizations for 2016, which contributed to the decrease in Alliant Energy’s and WPL’s “Energy efficiency cost recovery” regulatory liabilities. Utility Rate Cases - WPL’s Wisconsin Retail Electric and Gas Rate Case (2017/2018 Test Period) - In December 2016, WPL received an order from the PSCW authorizing WPL to implement increases in annual retail electric and gas base rates of $9 million and $9 million, respectively, effective January 1, 2017 followed by a freeze of such rates through the end of 2018. The key drivers for the electric and gas base rate increases are recovery of the costs for environmental controls projects at Edgewater and Columbia, and investments in electric and gas distribution systems, including expansion of natural gas pipeline infrastructure. The filing also included utilization of amounts that WPL previously over-recovered from its customers for energy efficiency cost recovery and electric transmission cost recovery, as well as amounts deferred under the return on common equity sharing mechanism for the 2013/2014 Test Period to reduce the requested base rate increases. 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 in 2017 or 2018. 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 then current levels through the end of 2016. The retail electric base rate case included a return of and return of costs for environmental controls projects, generation performance and reliability improvements, 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 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 required WPL to defer a portion of its earnings if its annual regulatory return on common equity exceeded certain levels in 2015 or 2016. As of December 31, 2016, Alliant Energy and WPL deferred $6 million of WPL’s 2016 earnings for these provisions, which is included in “Other” in Alliant Energy’s and WPL’s regulatory liabilities tables above. WPL’s Wisconsin Retail Electric and Gas Rate Case (2013/2014 Test Period) - The PSCW’s order for WPL’s 2013/2014 Test Period electric and gas base rate case included provisions 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, 2016, Alliant Energy and WPL deferred $6 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. These deferred earnings will be returned to customers as an offset to revenue requirements in 2017 and 2018 as authorized by the PSCW. 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 extended IPL’s Iowa retail electric base rates authorized in its 2009 Test Year rate case through 2016 and provided targeted retail electric customer billing credits of $105 million in aggregate. IPL recorded such billing credits as follows (in millions):
| 2016 |  | 2015 |  | 2014
Billing credits to reduce retail electric customers’ bills |  | $9 |  |  |  | $24 |  |  |  | $72 |
WPL’s Retail Fuel-related Rate Filings - The PSCW authorized annual retail electric rate increases for WPL in 2014, 2015 and 2016, resulting from anticipated increases in retail electric fuel-related costs during such periods. Retail fuel-related costs incurred by WPL in 2015 and 2016 were lower than fuel-related costs used to determine rates for such period resulting in an over-collection of fuel-related costs, and retail fuel-related costs incurred by WPL in 2014 were higher than fuel-related costs used to determine rates for such period resulting in an under-collection of fuel-related costs. These fuel-related costs were subject to deferral since they were outside an annual bandwidth of plus or minus 2% of the approved respective annual forecasted fuel-related costs. Details on these rate increases, as well as amounts WPL deferred for the over-collected (included in “Commodity cost recovery” regulatory liabilities) or under-collected (included in “Commodity cost recovery” regulatory assets) fuel-related costs from its retail electric customers are as follows (dollars in millions):
|  | Retail Electric |  | Deferral of Over (Under) |  | Timing of Refunds To or
Year |  | Rate Increase |  | Collected Fuel-related Costs |  | Collections From Customers
2016 |  |  | $7 |  |  | 1% |  |  | $9 |  |  | Pending
2015 |  | 39 |  |  | 4% |  | 10 |  |  | 2016
2014 |  | 19 |  |  | 2% |  | (28 | ) |  | 2016
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
| 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015
Tax-related |  | $1,055.6 |  |  |  | $987.7 |  |  |  | $1,022.4 |  |  |  | $958.2 |  |  |  | $33.2 |  |  |  | $29.5 |
Pension and OPEB costs | 578.7 |  |  | 579.5 |  |  | 294.0 |  |  | 298.1 |  |  | 284.7 |  |  | 281.4 |
AROs | 105.9 |  |  | 92.4 |  |  | 64.3 |  |  | 50.8 |  |  | 41.6 |  |  | 41.6 |
WPL’s EGUs retired early | 41.4 |  |  | 45.0 |  |  | — |  |  | — |  |  | 41.4 |  |  | 45.0 |
Derivatives | 30.7 |  |  | 70.6 |  |  | 10.0 |  |  | 28.2 |  |  | 20.7 |  |  | 42.4 |
Emission allowances | 26.2 |  |  | 26.9 |  |  | 26.2 |  |  | 26.9 |  |  | — |  |  | — |
Commodity cost recovery | 6.0 |  |  | 35.9 |  |  | 0.3 |  |  | 2.8 |  |  | 5.7 |  |  | 33.1 |
Other | 70.6 |  |  | 70.6 |  |  | 41.6 |  |  | 37.6 |  |  | 29.0 |  |  | 33.0 |
|  | $1,915.1 |  |  |  | $1,908.6 |  |  |  | $1,458.8 |  |  |  | $1,402.6 |  |  |  | $456.3 |  |  |  | $506.0 |
A portion of the regulatory assets in the above table are not earning a return. These regulatory assets, but not the respective carrying costs of these regulatory assets, are expected to be recovered from customers in future rates. At December 31, 2016, IPL and WPL had $44 million and $9 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 emission allowances, debt redemption costs, and costs for clean air compliance and wind generation expansion projects. 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 for IPL include the impacts of qualifying deductions for repairs expenditures, 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 2016, Alliant Energy’s and IPL’s tax-related regulatory assets increased primarily due to property-related differences for qualifying repair expenditures. 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 amounts 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 IPL’s and WPL’s respective regulatory jurisdictions. 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 has authorized WPL to recover from its retail electric and gas customers an estimated allocated portion of annual costs equal to the costs expected to be incurred during each test period. IPL and WPL are authorized to recover from their wholesale customers an allocated portion of actual pension costs incurred each year through FERC-approved formula 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. 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 the PSCW and FERC 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 PSCW and FERC orders. 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. 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. Alliant Energy and IPL have recorded a regulatory asset for amounts paid under the forward contracts, as well as for the remaining obligation under the forward contracts as a result of concluding the amount is probable of recovery from IPL’s customers. Commodity cost recovery - Refer to Note 1(g) for additional details of IPL’s and WPL’s cost recovery mechanisms. 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, 2016 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
| 2016 |  | 2015 |  | 2016 |  | 2015 |  | 2016 |  | 2015
Cost of removal obligations |  | $411.6 |  |  |  | $406.0 |  |  |  | $269.4 |  |  |  | $260.4 |  |  |  | $142.2 |  |  |  | $145.6 |
IPL’s tax benefit riders | 83.5 |  |  | 159.2 |  |  | 83.5 |  |  | 159.2 |  |  | — |  |  | — |
Electric transmission cost recovery | 72.0 |  |  | 43.5 |  |  | 35.7 |  |  | 21.9 |  |  | 36.3 |  |  | 21.6 |
Derivatives | 31.5 |  |  | 8.5 |  |  | 12.1 |  |  | 6.7 |  |  | 19.4 |  |  | 1.8 |
Commodity cost recovery | 30.8 |  |  | 37.6 |  |  | 17.8 |  |  | 23.5 |  |  | 13.0 |  |  | 14.1 |
Energy efficiency cost recovery | 20.5 |  |  | 48.3 |  |  | — |  |  | — |  |  | 20.5 |  |  | 48.3 |
Other | 31.1 |  |  | 34.6 |  |  | 12.3 |  |  | 17.5 |  |  | 18.8 |  |  | 17.1 |
|  | $681.0 |  |  |  | $737.7 |  |  |  | $430.8 |  |  |  | $489.2 |  |  |  | $250.2 |  |  |  | $248.5 |
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 - The IUB has approved electric and gas tax benefit riders proposed by IPL, which 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. Alliant Energy and IPL recognize an offsetting reduction to income tax expense for the after-tax amounts credited to such customers, resulting in no impact to their net income from the electric and gas tax benefit riders. The tax benefit riders 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 2016, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities decreased by $76 million as follows (in millions):
Electric tax benefit rider credits |  | $64 |
Gas tax benefit rider credits | 12 |
|  | $76 |
In December 2016, the IUB authorized $68 million and $7 million of regulatory liabilities from tax benefits to be credited to IPL’s retail electric and gas customers’ bills in 2017 through the electric and gas tax benefit riders, respectively. Any remaining tax benefit rider regulatory liabilities are currently expected to be credited to IPL’s retail electric and gas customers’ bills in the future, subject to final review by the IUB. Electric tax benefit rider - Details for IPL’s electric tax benefit rider are as follows (in millions):
| 2016 |  | 2015 |  | 2014
Credit to IPL’s Iowa retail electric customers’ bills with reduction to electric revenues (based on customers’ KWh usage) |  | $64 |  |  |  | $72 |  |  |  | $85 |
Income tax benefit resulting from decreased taxable income caused by credits | 27 |  |  | 30 |  |  | 35 |
Income tax benefit representing tax benefits realized from electric tax benefit rider | 37 |  |  | 42 |  |  | 50 |
The IUB authorized IPL to reduce the electric tax benefit rider billing credits on customers’ bills for 2013 through 2016 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):
| 2016 |  | 2015 |  | 2014
Revenue requirement adjustment |  | $14 |  |  |  | $14 |  |  |  | $15 |
Gas tax benefit rider - Details for IPL’s gas tax benefit rider are as follows (in millions):
| 2016 |  | 2015 |  | 2014
Credit to IPL’s Iowa retail gas customers’ bills with reduction to gas revenues (based on a fixed amount per day) |  | $12 |  |  |  | $12 |  |  |  | $12 |
Income tax benefit resulting from decreased taxable income caused by credits | 5 |  |  | 5 |  |  | 5 |
Income tax benefit representing tax benefits realized from gas tax benefit rider | 7 |  |  | 7 |  |  | 7 |
Electric transmission cost recovery - 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. Energy efficiency cost recovery - WPL and IPL collect revenues from their customers to offset certain expenditures they each incur 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 PSCW’s order for WPL’s 2015/2016 Test Period electric and gas base rate case authorized energy efficiency cost recovery amortizations for 2016, which contributed to the decrease in Alliant Energy’s and WPL’s “Energy efficiency cost recovery” regulatory liabilities. Utility Rate Cases - WPL’s Wisconsin Retail Electric and Gas Rate Case (2017/2018 Test Period) - In December 2016, WPL received an order from the PSCW authorizing WPL to implement increases in annual retail electric and gas base rates of $9 million and $9 million, respectively, effective January 1, 2017 followed by a freeze of such rates through the end of 2018. The key drivers for the electric and gas base rate increases are recovery of the costs for environmental controls projects at Edgewater and Columbia, and investments in electric and gas distribution systems, including expansion of natural gas pipeline infrastructure. The filing also included utilization of amounts that WPL previously over-recovered from its customers for energy efficiency cost recovery and electric transmission cost recovery, as well as amounts deferred under the return on common equity sharing mechanism for the 2013/2014 Test Period to reduce the requested base rate increases. 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 in 2017 or 2018. 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 then current levels through the end of 2016. The retail electric base rate case included a return of and return of costs for environmental controls projects, generation performance and reliability improvements, 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 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 required WPL to defer a portion of its earnings if its annual regulatory return on common equity exceeded certain levels in 2015 or 2016. As of December 31, 2016, Alliant Energy and WPL deferred $6 million of WPL’s 2016 earnings for these provisions, which is included in “Other” in Alliant Energy’s and WPL’s regulatory liabilities tables above. WPL’s Wisconsin Retail Electric and Gas Rate Case (2013/2014 Test Period) - The PSCW’s order for WPL’s 2013/2014 Test Period electric and gas base rate case included provisions 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, 2016, Alliant Energy and WPL deferred $6 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. These deferred earnings will be returned to customers as an offset to revenue requirements in 2017 and 2018 as authorized by the PSCW. 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 extended IPL’s Iowa retail electric base rates authorized in its 2009 Test Year rate case through 2016 and provided targeted retail electric customer billing credits of $105 million in aggregate. IPL recorded such billing credits as follows (in millions):
| 2016 |  | 2015 |  | 2014
Billing credits to reduce retail electric customers’ bills |  | $9 |  |  |  | $24 |  |  |  | $72 |
WPL’s Retail Fuel-related Rate Filings - The PSCW authorized annual retail electric rate increases for WPL in 2014, 2015 and 2016, resulting from anticipated increases in retail electric fuel-related costs during such periods. Retail fuel-related costs incurred by WPL in 2015 and 2016 were lower than fuel-related costs used to determine rates for such period resulting in an over-collection of fuel-related costs, and retail fuel-related costs incurred by WPL in 2014 were higher than fuel-related costs used to determine rates for such period resulting in an under-collection of fuel-related costs. These fuel-related costs were subject to deferral since they were outside an annual bandwidth of plus or minus 2% of the approved respective annual forecasted fuel-related costs. Details on these rate increases, as well as amounts WPL deferred for the over-collected (included in “Commodity cost recovery” regulatory liabilities) or under-collected (included in “Commodity cost recovery” regulatory assets) fuel-related costs from its retail electric customers are as follows (dollars in millions):
|  | Retail Electric |  | Deferral of Over (Under) |  | Timing of Refunds To or
Year |  | Rate Increase |  | Collected Fuel-related Costs |  | Collections From Customers
2016 |  |  | $7 |  |  | 1% |  |  | $9 |  |  | Pending
2015 |  | 39 |  |  | 4% |  | 10 |  |  | 2016
2014 |  | 19 |  |  | 2% |  | (28 | ) |  | 2016

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