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Regulatory Matters | 12 Months Ended
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Dec. 31, 2017
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
| 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Tax-related |  | $778.2 |  |  |  | $1,055.6 |  |  |  | $750.5 |  |  |  | $1,022.4 |  |  |  | $27.7 |  |  |  | $33.2 |
Pension and OPEB costs | 548.0 |  |  | 578.7 |  |  | 274.4 |  |  | 294.0 |  |  | 273.6 |  |  | 284.7 |
AROs | 109.3 |  |  | 105.9 |  |  | 72.5 |  |  | 64.3 |  |  | 36.8 |  |  | 41.6 |
EGUs retired early | 63.8 |  |  | 41.4 |  |  | 31.6 |  |  | — |  |  | 32.2 |  |  | 41.4 |
Derivatives | 45.3 |  |  | 30.7 |  |  | 21.8 |  |  | 10.0 |  |  | 23.5 |  |  | 20.7 |
Emission allowances | 25.5 |  |  | 26.2 |  |  | 25.5 |  |  | 26.2 |  |  | — |  |  | — |
Other | 96.6 |  |  | 76.6 |  |  | 55.3 |  |  | 41.9 |  |  | 41.3 |  |  | 34.7 |
|  | $1,666.7 |  |  |  | $1,915.1 |  |  |  | $1,231.6 |  |  |  | $1,458.8 |  |  |  | $435.1 |  |  |  | $456.3 |
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, 2017, IPL and WPL had $56 million and $5 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 environmental-related costs, and amounts related to the wholesale portion of under-collected fuel-related costs, which is discussed in Note 1(g). 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 latter 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. Tax-related regulatory assets are classified as non-current. During 2017, Alliant Energy’s, IPL’s and WPL’s tax-related regulatory assets decreased primarily due to the impacts of Tax Reform, which is discussed in Note 11. As a result of the reduced tax rate from Tax Reform, the amount of taxes needed to be collected from customers in the future has decreased, which resulted in a corresponding decrease in the associated regulatory assets. 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. In February 2018, the IUB authorized IPL to recover from its Iowa retail electric customers an allocated portion of annual costs equal to a five-year inflation-adjusted average of actual costs incurred for 2013 through 2016 and an estimate of costs for its forward-looking post-Test Year (2017). 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 utility operations and expenses recovered currently in rates will be recoverable in future rates, and are deferring the differences as regulatory assets. IPL’s settlement reached in September 2017 and approved by the IUB in February 2018 did not include the recovery of certain ARO costs previously recorded as regulatory assets, and as a result, Alliant Energy and IPL recorded a write-down of regulatory assets in 2017 as discussed in “IPL’s Retail Electric Rate Review (2016 Test Year)” below. Refer to Note 13 for additional details of AROs. Electric generating units retired early - In June 2017, IPL retired Sutherland Units 1 and 3 and reclassified the remaining net book value of these EGUs from property, plant and equipment to a regulatory asset on Alliant Energy’s and IPL’s balance sheets. IPL was earning a return on the remaining net book value of these EGUs, as well as recovering the remaining net book value of these EGUs from both its retail and wholesale customers. IPL’s settlement reached in September 2017 and approved by the IUB in February 2018 authorized IPL to recover the remaining net book value of these EGUs from IPL’s retail customers over a 10-year period; however, IPL is not allowed to earn a return on the remaining net book value of these EGUs from its retail customers. As a result, Alliant Energy and IPL recorded a write-down of regulatory assets in 2017 as discussed in “IPL’s Retail Electric Rate Review (2016 Test Year)” below. In September 2017, FERC approved continued recovery of the remaining net book value of these EGUs from IPL’s wholesale customers over a 10-year period. In 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 expected future emission reduction standards. Alliant Energy and IPL have recorded a regulatory asset for amounts paid under the forward contracts. In February 2018, the IUB authorized IPL to recover the unamortized forward contract costs for SO2 emission allowances through the energy adjustment clause over a 10-year period. Other - Alliant Energy, IPL and WPL assess whether IPL’s and WPL’s regulatory assets are probable of future recovery by considering factors such as applicable regulations, recent orders by the applicable regulatory agencies, historical treatment of similar costs by the applicable regulatory agencies and regulatory environment changes. Based on these assessments, Alliant Energy, IPL and WPL believe the regulatory assets recognized as of December 31, 2017 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
| 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Tax-related |  | $899.4 |  |  |  | $7.7 |  |  |  | $399.5 |  |  |  | $1.9 |  |  |  | $499.9 |  |  |  | $5.8 |
Cost of removal obligations | 410.0 |  |  | 411.6 |  |  | 274.5 |  |  | 269.4 |  |  | 135.5 |  |  | 142.2 |
Electric transmission cost recovery | 90.4 |  |  | 72.0 |  |  | 26.4 |  |  | 35.7 |  |  | 64.0 |  |  | 36.3 |
IPL’s tax benefit riders | 25.0 |  |  | 83.5 |  |  | 25.0 |  |  | 83.5 |  |  | — |  |  | — |
Commodity cost recovery | 21.0 |  |  | 30.8 |  |  | 14.6 |  |  | 17.8 |  |  | 6.4 |  |  | 13.0 |
Energy efficiency cost recovery | 19.9 |  |  | 20.5 |  |  | — |  |  | — |  |  | 19.9 |  |  | 20.5 |
Other | 31.5 |  |  | 54.9 |  |  | 15.4 |  |  | 22.5 |  |  | 16.1 |  |  | 32.4 |
|  | $1,497.2 |  |  |  | $681.0 |  |  |  | $755.4 |  |  |  | $430.8 |  |  |  | $741.8 |  |  |  | $250.2 |
Regulatory liabilities related to cost of removal obligations, to the extent expensed through depreciation rates, reduce rate base. Excluding the regulatory liabilities arising from Tax Reform, 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. Tax-related - During 2017, Alliant Energy’s, IPL’s and WPL’s tax-related regulatory liabilities increased primarily due to the impacts of Tax Reform, which is discussed in Note 11. Tax-related regulatory liabilities are classified as non-current. 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. Electric transmission cost recovery - A group of MISO cooperative and municipal utilities previously filed two complaints with FERC requesting a reduction to the base return on equity used by MISO transmission owners, including ITC and ATC, to determine electric transmission costs billed to utilities, including IPL and WPL. In 2016, FERC issued an order on the first complaint and established a base return on equity of 10.32%, excluding any incentive adders granted by FERC. The base return on equity of 10.32% was effective September 28, 2016, and was for the refund period from November 12, 2013 through February 11, 2015 (first complaint period). In 2017, Alliant Energy, IPL and WPL received the refunds for the first complaint period of $50 million, $39 million and $11 million, respectively, after final true-ups. IPL and WPL each initially recorded the retail portion of the refunds to a regulatory liability. Pursuant to IUB approval, IPL’s retail portion of the refund from ITC was refunded to its retail customers in 2017. WPL’s retail portion of the refund from ATC will remain in a regulatory liability until such refunds are approved to be returned to retail customers in a future rate proceeding. IPL’s and WPL’s wholesale customers received their share of the refunds through normal monthly billing practices in 2017. The second complaint covers the period from February 12, 2015 through May 11, 2016. A decision from FERC on the second complaint is currently expected in 2018. Refer to Note 1(g) for additional details of IPL’s and WPL’s electric transmission service cost recovery mechanisms. 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 and cost of removal expenditures, and a rate-making accounting change for capitalized interest. In 2017, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities increased (decreased) by ($59) million as follows (in millions):
Electric tax benefit rider credits |  | ($65 | )
Gas tax benefit rider credits | (6 | )
Rate-making accounting change for capitalized interest | 17 |
Tax Reform adjustment (Refer to Note 11) | (5 | )
|  | ($59 | )
In 2017, Alliant Energy and IPL implemented a rate-making accounting change for capitalized interest. IPL currently anticipates crediting its related tax benefits from this rate-making accounting change to its Iowa retail electric and gas customers in the future, and as a result, Alliant Energy and IPL recorded an increase of $17 million to IPL’s tax benefit riders regulatory liabilities in 2017. The remaining electric tax benefit rider regulatory liabilities are currently expected to be credited to IPL’s retail electric customers’ bills over a 12-month period after final rates are effective, which is currently expected in the first half of 2018. The remaining gas tax benefit rider regulatory liabilities are currently expected to be credited to IPL’s gas customers’ bills by December 2018, subject to final review by the IUB. Electric tax benefit rider - Details for IPL’s electric tax benefit rider are as follows (in millions):
| 2017 |  | 2016 |  | 2015
Credit to IPL’s Iowa retail electric customers’ bills with reduction to electric revenues (based on customers’ KWh usage) |  | $65 |  |  |  | $64 |  |  |  | $72 |
Income tax benefit resulting from decreased taxable income caused by credits | 27 |  |  | 27 |  |  | 30 |
Income tax benefit representing tax benefits realized from electric tax benefit rider | 38 |  |  | 37 |  |  | 42 |
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
Revenue requirement adjustment |  | $14 |  |  |  | $14 |
Gas tax benefit rider - Details for IPL’s gas tax benefit rider are as follows (in millions):
| 2017 |  | 2016 |  | 2015
Credit to IPL’s Iowa retail gas customers’ bills with reduction to gas revenues (based on a fixed amount per day) |  | $6 |  |  |  | $12 |  |  |  | $12 |
Income tax benefit resulting from decreased taxable income caused by credits | 3 |  |  | 5 |  |  | 5 |
Income tax benefit representing tax benefits realized from gas tax benefit rider | 3 |  |  | 7 |  |  | 7 |
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. Utility Rate Reviews - IPL’s Retail Electric Rate Review (2016 Test Year) - In April 2017, IPL filed a request with the IUB to increase annual electric base rates for its Iowa retail electric customers. The request was based on a 2016 historical Test Year as adjusted for certain known and measurable changes occurring up to 12 months after the commencement of the proceeding. The key drivers for the filing included recovery of capital projects, primarily power grid modernization and investments that advance cleaner energy, including Marshalltown. An interim retail electric base rate increase of $102 million, or approximately 7%, on an annual basis, was implemented effective April 13, 2017. Tax benefit rider credits and MISO transmission owner return on equity refunds were used to reduce the effect of the interim rate increase on customer bills in 2017. In 2017, Alliant Energy and IPL recorded increases in electric base rates of $77 million in conjunction with the interim retail electric base rate increase. In September 2017, IPL reached a partial, non-unanimous settlement agreement with the Iowa Office of Consumer Advocate, the Iowa Business Energy Coalition and the Large Energy Group for an annual retail electric base rate increase of $130 million, or approximately 9%. In February 2018, the IUB issued an order approving the settlement. Final rates are currently expected to be effective in the first half of 2018 once all motions for reconsideration have been addressed and final tariffs have been approved by the IUB. The final rate increase includes continuation of the electric transmission cost rider; increased depreciation expense resulting from an updated depreciation study; recovery over a four-year period of ARO expenditures since the last retail electric rate filing in 2010; recovery over a 10-year period of the remaining net book value of Sutherland Units 1 and 3, unamortized forward contract costs for SO2 emission allowances through the energy adjustment clause and cancelled project costs approved in a prior EPB; and no double leverage applied to the weighted-average cost of capital. As a result of the partial settlement, in 2017, IPL recorded a write-down of regulatory assets of $9 million, including $4 million to “Other operation and maintenance” expenses primarily related to IPL being no longer probable of earning a return on the remaining net book value of Sutherland Units 1 and 3 from its retail customers when final rates are implemented, and $5 million to “Depreciation and amortization” expenses for certain AROs deemed no longer probable of recovery in future rates. WPL’s Retail Electric and Gas Rate Review (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 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 $9 million net annual retail electric rate increase reflects a $60 million increase in base rates, partially offset by a $51 million reduction in fuel-related costs, using an estimate for 2017 fuel-related costs. 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 earnings 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. In 2017, Alliant Energy and WPL recorded increases in electric and gas base rates of $63 million and $9 million, respectively, in conjunction with the base rate increases authorized in the PSCW’s December 2016 order. WPL’s Retail Electric and Gas Rate Review (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 review included a return of and return on 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, 2017, Alliant Energy and WPL deferred $5 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. IPL’s 2014 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 review 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 Filing (2016 Test Year) - Pursuant to a 2015 PSCW order, WPL’s 2016 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 in 2016 were lower than fuel-related costs used to determine rates for such period resulting in an over-collection of fuel-related costs. In August 2017, the PSCW authorized WPL to utilize $6 million of the over-collections as an offset to projected 2017 fuel-related cost under-collections. As of December 31, 2017, $3 million of remaining fuel-related costs for 2016 outside of the approved bandwidth are included in “Commodity cost recovery” in Alliant Energy’s and WPL’s regulatory liabilities table above, and these costs are expected to offset any rate changes for WPL’s 2018 fuel-related costs. WPL’s Retail Fuel-related Rate Filing (2017 Test Year) - In March 2017, WPL filed an application with the PSCW for a mid-year fuel-related cost adjustment for 2017. Fuel-related costs for 2017 were expected to exceed the approved 2017 fuel-related cost plan by more than the 2% annual bandwidth. In August 2017, the PSCW authorized WPL to utilize $6 million of the 2016 fuel-related cost over-collections to offset a portion of the projected fuel-related cost under-collections for 2017. Retail fuel-related costs incurred by WPL in 2017 were higher than the projected fuel-related costs used to determine rates for such period resulting in an under-collection of fuel-related costs. As of December 31, 2017, after applying the 2016 over-recovery amounts, the remaining fuel-related costs for 2017 outside of the approved bandwidth were $5 million and are included in “Other” in Alliant Energy’s and WPL’s regulatory assets table above. WPL’s Retail Fuel-related Rate Filing (2018 Test Year) - In December 2017, WPL received an order from the PSCW authorizing an annual retail electric rate increase of $6 million, or approximately 1%, effective January 1, 2018. The increase primarily reflects a change in expected fuel-related costs in 2018, which was offset by $3 million of over-collections from WPL’s 2016 fuel-related costs as discussed above.
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
| 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Tax-related |  | $778.2 |  |  |  | $1,055.6 |  |  |  | $750.5 |  |  |  | $1,022.4 |  |  |  | $27.7 |  |  |  | $33.2 |
Pension and OPEB costs | 548.0 |  |  | 578.7 |  |  | 274.4 |  |  | 294.0 |  |  | 273.6 |  |  | 284.7 |
AROs | 109.3 |  |  | 105.9 |  |  | 72.5 |  |  | 64.3 |  |  | 36.8 |  |  | 41.6 |
EGUs retired early | 63.8 |  |  | 41.4 |  |  | 31.6 |  |  | — |  |  | 32.2 |  |  | 41.4 |
Derivatives | 45.3 |  |  | 30.7 |  |  | 21.8 |  |  | 10.0 |  |  | 23.5 |  |  | 20.7 |
Emission allowances | 25.5 |  |  | 26.2 |  |  | 25.5 |  |  | 26.2 |  |  | — |  |  | — |
Other | 96.6 |  |  | 76.6 |  |  | 55.3 |  |  | 41.9 |  |  | 41.3 |  |  | 34.7 |
|  | $1,666.7 |  |  |  | $1,915.1 |  |  |  | $1,231.6 |  |  |  | $1,458.8 |  |  |  | $435.1 |  |  |  | $456.3 |
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, 2017, IPL and WPL had $56 million and $5 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 environmental-related costs, and amounts related to the wholesale portion of under-collected fuel-related costs, which is discussed in Note 1(g). 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 latter 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. Tax-related regulatory assets are classified as non-current. During 2017, Alliant Energy’s, IPL’s and WPL’s tax-related regulatory assets decreased primarily due to the impacts of Tax Reform, which is discussed in Note 11. As a result of the reduced tax rate from Tax Reform, the amount of taxes needed to be collected from customers in the future has decreased, which resulted in a corresponding decrease in the associated regulatory assets. 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. In February 2018, the IUB authorized IPL to recover from its Iowa retail electric customers an allocated portion of annual costs equal to a five-year inflation-adjusted average of actual costs incurred for 2013 through 2016 and an estimate of costs for its forward-looking post-Test Year (2017). 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 utility operations and expenses recovered currently in rates will be recoverable in future rates, and are deferring the differences as regulatory assets. IPL’s settlement reached in September 2017 and approved by the IUB in February 2018 did not include the recovery of certain ARO costs previously recorded as regulatory assets, and as a result, Alliant Energy and IPL recorded a write-down of regulatory assets in 2017 as discussed in “IPL’s Retail Electric Rate Review (2016 Test Year)” below. Refer to Note 13 for additional details of AROs. Electric generating units retired early - In June 2017, IPL retired Sutherland Units 1 and 3 and reclassified the remaining net book value of these EGUs from property, plant and equipment to a regulatory asset on Alliant Energy’s and IPL’s balance sheets. IPL was earning a return on the remaining net book value of these EGUs, as well as recovering the remaining net book value of these EGUs from both its retail and wholesale customers. IPL’s settlement reached in September 2017 and approved by the IUB in February 2018 authorized IPL to recover the remaining net book value of these EGUs from IPL’s retail customers over a 10-year period; however, IPL is not allowed to earn a return on the remaining net book value of these EGUs from its retail customers. As a result, Alliant Energy and IPL recorded a write-down of regulatory assets in 2017 as discussed in “IPL’s Retail Electric Rate Review (2016 Test Year)” below. In September 2017, FERC approved continued recovery of the remaining net book value of these EGUs from IPL’s wholesale customers over a 10-year period. In 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 expected future emission reduction standards. Alliant Energy and IPL have recorded a regulatory asset for amounts paid under the forward contracts. In February 2018, the IUB authorized IPL to recover the unamortized forward contract costs for SO2 emission allowances through the energy adjustment clause over a 10-year period. Other - Alliant Energy, IPL and WPL assess whether IPL’s and WPL’s regulatory assets are probable of future recovery by considering factors such as applicable regulations, recent orders by the applicable regulatory agencies, historical treatment of similar costs by the applicable regulatory agencies and regulatory environment changes. Based on these assessments, Alliant Energy, IPL and WPL believe the regulatory assets recognized as of December 31, 2017 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
| 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Tax-related |  | $899.4 |  |  |  | $7.7 |  |  |  | $399.5 |  |  |  | $1.9 |  |  |  | $499.9 |  |  |  | $5.8 |
Cost of removal obligations | 410.0 |  |  | 411.6 |  |  | 274.5 |  |  | 269.4 |  |  | 135.5 |  |  | 142.2 |
Electric transmission cost recovery | 90.4 |  |  | 72.0 |  |  | 26.4 |  |  | 35.7 |  |  | 64.0 |  |  | 36.3 |
IPL’s tax benefit riders | 25.0 |  |  | 83.5 |  |  | 25.0 |  |  | 83.5 |  |  | — |  |  | — |
Commodity cost recovery | 21.0 |  |  | 30.8 |  |  | 14.6 |  |  | 17.8 |  |  | 6.4 |  |  | 13.0 |
Energy efficiency cost recovery | 19.9 |  |  | 20.5 |  |  | — |  |  | — |  |  | 19.9 |  |  | 20.5 |
Other | 31.5 |  |  | 54.9 |  |  | 15.4 |  |  | 22.5 |  |  | 16.1 |  |  | 32.4 |
|  | $1,497.2 |  |  |  | $681.0 |  |  |  | $755.4 |  |  |  | $430.8 |  |  |  | $741.8 |  |  |  | $250.2 |
Regulatory liabilities related to cost of removal obligations, to the extent expensed through depreciation rates, reduce rate base. Excluding the regulatory liabilities arising from Tax Reform, 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. Tax-related - During 2017, Alliant Energy’s, IPL’s and WPL’s tax-related regulatory liabilities increased primarily due to the impacts of Tax Reform, which is discussed in Note 11. Tax-related regulatory liabilities are classified as non-current. 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. Electric transmission cost recovery - A group of MISO cooperative and municipal utilities previously filed two complaints with FERC requesting a reduction to the base return on equity used by MISO transmission owners, including ITC and ATC, to determine electric transmission costs billed to utilities, including IPL and WPL. In 2016, FERC issued an order on the first complaint and established a base return on equity of 10.32%, excluding any incentive adders granted by FERC. The base return on equity of 10.32% was effective September 28, 2016, and was for the refund period from November 12, 2013 through February 11, 2015 (first complaint period). In 2017, Alliant Energy, IPL and WPL received the refunds for the first complaint period of $50 million, $39 million and $11 million, respectively, after final true-ups. IPL and WPL each initially recorded the retail portion of the refunds to a regulatory liability. Pursuant to IUB approval, IPL’s retail portion of the refund from ITC was refunded to its retail customers in 2017. WPL’s retail portion of the refund from ATC will remain in a regulatory liability until such refunds are approved to be returned to retail customers in a future rate proceeding. IPL’s and WPL’s wholesale customers received their share of the refunds through normal monthly billing practices in 2017. The second complaint covers the period from February 12, 2015 through May 11, 2016. A decision from FERC on the second complaint is currently expected in 2018. Refer to Note 1(g) for additional details of IPL’s and WPL’s electric transmission service cost recovery mechanisms. 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 and cost of removal expenditures, and a rate-making accounting change for capitalized interest. In 2017, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities increased (decreased) by ($59) million as follows (in millions):
Electric tax benefit rider credits |  | ($65 | )
Gas tax benefit rider credits | (6 | )
Rate-making accounting change for capitalized interest | 17 |
Tax Reform adjustment (Refer to Note 11) | (5 | )
|  | ($59 | )
In 2017, Alliant Energy and IPL implemented a rate-making accounting change for capitalized interest. IPL currently anticipates crediting its related tax benefits from this rate-making accounting change to its Iowa retail electric and gas customers in the future, and as a result, Alliant Energy and IPL recorded an increase of $17 million to IPL’s tax benefit riders regulatory liabilities in 2017. The remaining electric tax benefit rider regulatory liabilities are currently expected to be credited to IPL’s retail electric customers’ bills over a 12-month period after final rates are effective, which is currently expected in the first half of 2018. The remaining gas tax benefit rider regulatory liabilities are currently expected to be credited to IPL’s gas customers’ bills by December 2018, subject to final review by the IUB. Electric tax benefit rider - Details for IPL’s electric tax benefit rider are as follows (in millions):
| 2017 |  | 2016 |  | 2015
Credit to IPL’s Iowa retail electric customers’ bills with reduction to electric revenues (based on customers’ KWh usage) |  | $65 |  |  |  | $64 |  |  |  | $72 |
Income tax benefit resulting from decreased taxable income caused by credits | 27 |  |  | 27 |  |  | 30 |
Income tax benefit representing tax benefits realized from electric tax benefit rider | 38 |  |  | 37 |  |  | 42 |
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
Revenue requirement adjustment |  | $14 |  |  |  | $14 |
Gas tax benefit rider - Details for IPL’s gas tax benefit rider are as follows (in millions):
| 2017 |  | 2016 |  | 2015
Credit to IPL’s Iowa retail gas customers’ bills with reduction to gas revenues (based on a fixed amount per day) |  | $6 |  |  |  | $12 |  |  |  | $12 |
Income tax benefit resulting from decreased taxable income caused by credits | 3 |  |  | 5 |  |  | 5 |
Income tax benefit representing tax benefits realized from gas tax benefit rider | 3 |  |  | 7 |  |  | 7 |
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. Utility Rate Reviews - IPL’s Retail Electric Rate Review (2016 Test Year) - In April 2017, IPL filed a request with the IUB to increase annual electric base rates for its Iowa retail electric customers. The request was based on a 2016 historical Test Year as adjusted for certain known and measurable changes occurring up to 12 months after the commencement of the proceeding. The key drivers for the filing included recovery of capital projects, primarily power grid modernization and investments that advance cleaner energy, including Marshalltown. An interim retail electric base rate increase of $102 million, or approximately 7%, on an annual basis, was implemented effective April 13, 2017. Tax benefit rider credits and MISO transmission owner return on equity refunds were used to reduce the effect of the interim rate increase on customer bills in 2017. In 2017, Alliant Energy and IPL recorded increases in electric base rates of $77 million in conjunction with the interim retail electric base rate increase. In September 2017, IPL reached a partial, non-unanimous settlement agreement with the Iowa Office of Consumer Advocate, the Iowa Business Energy Coalition and the Large Energy Group for an annual retail electric base rate increase of $130 million, or approximately 9%. In February 2018, the IUB issued an order approving the settlement. Final rates are currently expected to be effective in the first half of 2018 once all motions for reconsideration have been addressed and final tariffs have been approved by the IUB. The final rate increase includes continuation of the electric transmission cost rider; increased depreciation expense resulting from an updated depreciation study; recovery over a four-year period of ARO expenditures since the last retail electric rate filing in 2010; recovery over a 10-year period of the remaining net book value of Sutherland Units 1 and 3, unamortized forward contract costs for SO2 emission allowances through the energy adjustment clause and cancelled project costs approved in a prior EPB; and no double leverage applied to the weighted-average cost of capital. As a result of the partial settlement, in 2017, IPL recorded a write-down of regulatory assets of $9 million, including $4 million to “Other operation and maintenance” expenses primarily related to IPL being no longer probable of earning a return on the remaining net book value of Sutherland Units 1 and 3 from its retail customers when final rates are implemented, and $5 million to “Depreciation and amortization” expenses for certain AROs deemed no longer probable of recovery in future rates. WPL’s Retail Electric and Gas Rate Review (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 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 $9 million net annual retail electric rate increase reflects a $60 million increase in base rates, partially offset by a $51 million reduction in fuel-related costs, using an estimate for 2017 fuel-related costs. 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 earnings 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. In 2017, Alliant Energy and WPL recorded increases in electric and gas base rates of $63 million and $9 million, respectively, in conjunction with the base rate increases authorized in the PSCW’s December 2016 order. WPL’s Retail Electric and Gas Rate Review (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 review included a return of and return on 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, 2017, Alliant Energy and WPL deferred $5 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. IPL’s 2014 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 review 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 Filing (2016 Test Year) - Pursuant to a 2015 PSCW order, WPL’s 2016 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 in 2016 were lower than fuel-related costs used to determine rates for such period resulting in an over-collection of fuel-related costs. In August 2017, the PSCW authorized WPL to utilize $6 million of the over-collections as an offset to projected 2017 fuel-related cost under-collections. As of December 31, 2017, $3 million of remaining fuel-related costs for 2016 outside of the approved bandwidth are included in “Commodity cost recovery” in Alliant Energy’s and WPL’s regulatory liabilities table above, and these costs are expected to offset any rate changes for WPL’s 2018 fuel-related costs. WPL’s Retail Fuel-related Rate Filing (2017 Test Year) - In March 2017, WPL filed an application with the PSCW for a mid-year fuel-related cost adjustment for 2017. Fuel-related costs for 2017 were expected to exceed the approved 2017 fuel-related cost plan by more than the 2% annual bandwidth. In August 2017, the PSCW authorized WPL to utilize $6 million of the 2016 fuel-related cost over-collections to offset a portion of the projected fuel-related cost under-collections for 2017. Retail fuel-related costs incurred by WPL in 2017 were higher than the projected fuel-related costs used to determine rates for such period resulting in an under-collection of fuel-related costs. As of December 31, 2017, after applying the 2016 over-recovery amounts, the remaining fuel-related costs for 2017 outside of the approved bandwidth were $5 million and are included in “Other” in Alliant Energy’s and WPL’s regulatory assets table above. WPL’s Retail Fuel-related Rate Filing (2018 Test Year) - In December 2017, WPL received an order from the PSCW authorizing an annual retail electric rate increase of $6 million, or approximately 1%, effective January 1, 2018. The increase primarily reflects a change in expected fuel-related costs in 2018, which was offset by $3 million of over-collections from WPL’s 2016 fuel-related costs as discussed above.
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
| 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Tax-related |  | $778.2 |  |  |  | $1,055.6 |  |  |  | $750.5 |  |  |  | $1,022.4 |  |  |  | $27.7 |  |  |  | $33.2 |
Pension and OPEB costs | 548.0 |  |  | 578.7 |  |  | 274.4 |  |  | 294.0 |  |  | 273.6 |  |  | 284.7 |
AROs | 109.3 |  |  | 105.9 |  |  | 72.5 |  |  | 64.3 |  |  | 36.8 |  |  | 41.6 |
EGUs retired early | 63.8 |  |  | 41.4 |  |  | 31.6 |  |  | — |  |  | 32.2 |  |  | 41.4 |
Derivatives | 45.3 |  |  | 30.7 |  |  | 21.8 |  |  | 10.0 |  |  | 23.5 |  |  | 20.7 |
Emission allowances | 25.5 |  |  | 26.2 |  |  | 25.5 |  |  | 26.2 |  |  | — |  |  | — |
Other | 96.6 |  |  | 76.6 |  |  | 55.3 |  |  | 41.9 |  |  | 41.3 |  |  | 34.7 |
|  | $1,666.7 |  |  |  | $1,915.1 |  |  |  | $1,231.6 |  |  |  | $1,458.8 |  |  |  | $435.1 |  |  |  | $456.3 |
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, 2017, IPL and WPL had $56 million and $5 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 environmental-related costs, and amounts related to the wholesale portion of under-collected fuel-related costs, which is discussed in Note 1(g). 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 latter 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. Tax-related regulatory assets are classified as non-current. During 2017, Alliant Energy’s, IPL’s and WPL’s tax-related regulatory assets decreased primarily due to the impacts of Tax Reform, which is discussed in Note 11. As a result of the reduced tax rate from Tax Reform, the amount of taxes needed to be collected from customers in the future has decreased, which resulted in a corresponding decrease in the associated regulatory assets. 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. In February 2018, the IUB authorized IPL to recover from its Iowa retail electric customers an allocated portion of annual costs equal to a five-year inflation-adjusted average of actual costs incurred for 2013 through 2016 and an estimate of costs for its forward-looking post-Test Year (2017). 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 utility operations and expenses recovered currently in rates will be recoverable in future rates, and are deferring the differences as regulatory assets. IPL’s settlement reached in September 2017 and approved by the IUB in February 2018 did not include the recovery of certain ARO costs previously recorded as regulatory assets, and as a result, Alliant Energy and IPL recorded a write-down of regulatory assets in 2017 as discussed in “IPL’s Retail Electric Rate Review (2016 Test Year)” below. Refer to Note 13 for additional details of AROs. Electric generating units retired early - In June 2017, IPL retired Sutherland Units 1 and 3 and reclassified the remaining net book value of these EGUs from property, plant and equipment to a regulatory asset on Alliant Energy’s and IPL’s balance sheets. IPL was earning a return on the remaining net book value of these EGUs, as well as recovering the remaining net book value of these EGUs from both its retail and wholesale customers. IPL’s settlement reached in September 2017 and approved by the IUB in February 2018 authorized IPL to recover the remaining net book value of these EGUs from IPL’s retail customers over a 10-year period; however, IPL is not allowed to earn a return on the remaining net book value of these EGUs from its retail customers. As a result, Alliant Energy and IPL recorded a write-down of regulatory assets in 2017 as discussed in “IPL’s Retail Electric Rate Review (2016 Test Year)” below. In September 2017, FERC approved continued recovery of the remaining net book value of these EGUs from IPL’s wholesale customers over a 10-year period. In 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 expected future emission reduction standards. Alliant Energy and IPL have recorded a regulatory asset for amounts paid under the forward contracts. In February 2018, the IUB authorized IPL to recover the unamortized forward contract costs for SO2 emission allowances through the energy adjustment clause over a 10-year period. Other - Alliant Energy, IPL and WPL assess whether IPL’s and WPL’s regulatory assets are probable of future recovery by considering factors such as applicable regulations, recent orders by the applicable regulatory agencies, historical treatment of similar costs by the applicable regulatory agencies and regulatory environment changes. Based on these assessments, Alliant Energy, IPL and WPL believe the regulatory assets recognized as of December 31, 2017 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
| 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Tax-related |  | $899.4 |  |  |  | $7.7 |  |  |  | $399.5 |  |  |  | $1.9 |  |  |  | $499.9 |  |  |  | $5.8 |
Cost of removal obligations | 410.0 |  |  | 411.6 |  |  | 274.5 |  |  | 269.4 |  |  | 135.5 |  |  | 142.2 |
Electric transmission cost recovery | 90.4 |  |  | 72.0 |  |  | 26.4 |  |  | 35.7 |  |  | 64.0 |  |  | 36.3 |
IPL’s tax benefit riders | 25.0 |  |  | 83.5 |  |  | 25.0 |  |  | 83.5 |  |  | — |  |  | — |
Commodity cost recovery | 21.0 |  |  | 30.8 |  |  | 14.6 |  |  | 17.8 |  |  | 6.4 |  |  | 13.0 |
Energy efficiency cost recovery | 19.9 |  |  | 20.5 |  |  | — |  |  | — |  |  | 19.9 |  |  | 20.5 |
Other | 31.5 |  |  | 54.9 |  |  | 15.4 |  |  | 22.5 |  |  | 16.1 |  |  | 32.4 |
|  | $1,497.2 |  |  |  | $681.0 |  |  |  | $755.4 |  |  |  | $430.8 |  |  |  | $741.8 |  |  |  | $250.2 |
Regulatory liabilities related to cost of removal obligations, to the extent expensed through depreciation rates, reduce rate base. Excluding the regulatory liabilities arising from Tax Reform, 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. Tax-related - During 2017, Alliant Energy’s, IPL’s and WPL’s tax-related regulatory liabilities increased primarily due to the impacts of Tax Reform, which is discussed in Note 11. Tax-related regulatory liabilities are classified as non-current. 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. Electric transmission cost recovery - A group of MISO cooperative and municipal utilities previously filed two complaints with FERC requesting a reduction to the base return on equity used by MISO transmission owners, including ITC and ATC, to determine electric transmission costs billed to utilities, including IPL and WPL. In 2016, FERC issued an order on the first complaint and established a base return on equity of 10.32%, excluding any incentive adders granted by FERC. The base return on equity of 10.32% was effective September 28, 2016, and was for the refund period from November 12, 2013 through February 11, 2015 (first complaint period). In 2017, Alliant Energy, IPL and WPL received the refunds for the first complaint period of $50 million, $39 million and $11 million, respectively, after final true-ups. IPL and WPL each initially recorded the retail portion of the refunds to a regulatory liability. Pursuant to IUB approval, IPL’s retail portion of the refund from ITC was refunded to its retail customers in 2017. WPL’s retail portion of the refund from ATC will remain in a regulatory liability until such refunds are approved to be returned to retail customers in a future rate proceeding. IPL’s and WPL’s wholesale customers received their share of the refunds through normal monthly billing practices in 2017. The second complaint covers the period from February 12, 2015 through May 11, 2016. A decision from FERC on the second complaint is currently expected in 2018. Refer to Note 1(g) for additional details of IPL’s and WPL’s electric transmission service cost recovery mechanisms. 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 and cost of removal expenditures, and a rate-making accounting change for capitalized interest. In 2017, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities increased (decreased) by ($59) million as follows (in millions):
Electric tax benefit rider credits |  | ($65 | )
Gas tax benefit rider credits | (6 | )
Rate-making accounting change for capitalized interest | 17 |
Tax Reform adjustment (Refer to Note 11) | (5 | )
|  | ($59 | )
In 2017, Alliant Energy and IPL implemented a rate-making accounting change for capitalized interest. IPL currently anticipates crediting its related tax benefits from this rate-making accounting change to its Iowa retail electric and gas customers in the future, and as a result, Alliant Energy and IPL recorded an increase of $17 million to IPL’s tax benefit riders regulatory liabilities in 2017. The remaining electric tax benefit rider regulatory liabilities are currently expected to be credited to IPL’s retail electric customers’ bills over a 12-month period after final rates are effective, which is currently expected in the first half of 2018. The remaining gas tax benefit rider regulatory liabilities are currently expected to be credited to IPL’s gas customers’ bills by December 2018, subject to final review by the IUB. Electric tax benefit rider - Details for IPL’s electric tax benefit rider are as follows (in millions):
| 2017 |  | 2016 |  | 2015
Credit to IPL’s Iowa retail electric customers’ bills with reduction to electric revenues (based on customers’ KWh usage) |  | $65 |  |  |  | $64 |  |  |  | $72 |
Income tax benefit resulting from decreased taxable income caused by credits | 27 |  |  | 27 |  |  | 30 |
Income tax benefit representing tax benefits realized from electric tax benefit rider | 38 |  |  | 37 |  |  | 42 |
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
Revenue requirement adjustment |  | $14 |  |  |  | $14 |
Gas tax benefit rider - Details for IPL’s gas tax benefit rider are as follows (in millions):
| 2017 |  | 2016 |  | 2015
Credit to IPL’s Iowa retail gas customers’ bills with reduction to gas revenues (based on a fixed amount per day) |  | $6 |  |  |  | $12 |  |  |  | $12 |
Income tax benefit resulting from decreased taxable income caused by credits | 3 |  |  | 5 |  |  | 5 |
Income tax benefit representing tax benefits realized from gas tax benefit rider | 3 |  |  | 7 |  |  | 7 |
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. Utility Rate Reviews - IPL’s Retail Electric Rate Review (2016 Test Year) - In April 2017, IPL filed a request with the IUB to increase annual electric base rates for its Iowa retail electric customers. The request was based on a 2016 historical Test Year as adjusted for certain known and measurable changes occurring up to 12 months after the commencement of the proceeding. The key drivers for the filing included recovery of capital projects, primarily power grid modernization and investments that advance cleaner energy, including Marshalltown. An interim retail electric base rate increase of $102 million, or approximately 7%, on an annual basis, was implemented effective April 13, 2017. Tax benefit rider credits and MISO transmission owner return on equity refunds were used to reduce the effect of the interim rate increase on customer bills in 2017. In 2017, Alliant Energy and IPL recorded increases in electric base rates of $77 million in conjunction with the interim retail electric base rate increase. In September 2017, IPL reached a partial, non-unanimous settlement agreement with the Iowa Office of Consumer Advocate, the Iowa Business Energy Coalition and the Large Energy Group for an annual retail electric base rate increase of $130 million, or approximately 9%. In February 2018, the IUB issued an order approving the settlement. Final rates are currently expected to be effective in the first half of 2018 once all motions for reconsideration have been addressed and final tariffs have been approved by the IUB. The final rate increase includes continuation of the electric transmission cost rider; increased depreciation expense resulting from an updated depreciation study; recovery over a four-year period of ARO expenditures since the last retail electric rate filing in 2010; recovery over a 10-year period of the remaining net book value of Sutherland Units 1 and 3, unamortized forward contract costs for SO2 emission allowances through the energy adjustment clause and cancelled project costs approved in a prior EPB; and no double leverage applied to the weighted-average cost of capital. As a result of the partial settlement, in 2017, IPL recorded a write-down of regulatory assets of $9 million, including $4 million to “Other operation and maintenance” expenses primarily related to IPL being no longer probable of earning a return on the remaining net book value of Sutherland Units 1 and 3 from its retail customers when final rates are implemented, and $5 million to “Depreciation and amortization” expenses for certain AROs deemed no longer probable of recovery in future rates. WPL’s Retail Electric and Gas Rate Review (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 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 $9 million net annual retail electric rate increase reflects a $60 million increase in base rates, partially offset by a $51 million reduction in fuel-related costs, using an estimate for 2017 fuel-related costs. 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 earnings 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. In 2017, Alliant Energy and WPL recorded increases in electric and gas base rates of $63 million and $9 million, respectively, in conjunction with the base rate increases authorized in the PSCW’s December 2016 order. WPL’s Retail Electric and Gas Rate Review (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 review included a return of and return on 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, 2017, Alliant Energy and WPL deferred $5 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. IPL’s 2014 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 review 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 Filing (2016 Test Year) - Pursuant to a 2015 PSCW order, WPL’s 2016 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 in 2016 were lower than fuel-related costs used to determine rates for such period resulting in an over-collection of fuel-related costs. In August 2017, the PSCW authorized WPL to utilize $6 million of the over-collections as an offset to projected 2017 fuel-related cost under-collections. As of December 31, 2017, $3 million of remaining fuel-related costs for 2016 outside of the approved bandwidth are included in “Commodity cost recovery” in Alliant Energy’s and WPL’s regulatory liabilities table above, and these costs are expected to offset any rate changes for WPL’s 2018 fuel-related costs. WPL’s Retail Fuel-related Rate Filing (2017 Test Year) - In March 2017, WPL filed an application with the PSCW for a mid-year fuel-related cost adjustment for 2017. Fuel-related costs for 2017 were expected to exceed the approved 2017 fuel-related cost plan by more than the 2% annual bandwidth. In August 2017, the PSCW authorized WPL to utilize $6 million of the 2016 fuel-related cost over-collections to offset a portion of the projected fuel-related cost under-collections for 2017. Retail fuel-related costs incurred by WPL in 2017 were higher than the projected fuel-related costs used to determine rates for such period resulting in an under-collection of fuel-related costs. As of December 31, 2017, after applying the 2016 over-recovery amounts, the remaining fuel-related costs for 2017 outside of the approved bandwidth were $5 million and are included in “Other” in Alliant Energy’s and WPL’s regulatory assets table above. WPL’s Retail Fuel-related Rate Filing (2018 Test Year) - In December 2017, WPL received an order from the PSCW authorizing an annual retail electric rate increase of $6 million, or approximately 1%, effective January 1, 2018. The increase primarily reflects a change in expected fuel-related costs in 2018, which was offset by $3 million of over-collections from WPL’s 2016 fuel-related costs as discussed above.

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