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Regulatory Matters | 12 Months Ended
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Dec. 31, 2018
Regulatory Matters [Line Items] |
Regulatory Matters | REGULATORY MATTERS Regulatory Assets - 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, 2018 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. At December 31, regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Tax-related |  | $820.6 |  |  |  | $778.2 |  |  |  | $783.1 |  |  |  | $750.5 |  |  |  | $37.5 |  |  |  | $27.7 |
Pension and OPEB costs | 542.3 |  |  | 548.0 |  |  | 274.0 |  |  | 274.4 |  |  | 268.3 |  |  | 273.6 |
EGUs retired early | 111.6 |  |  | 63.8 |  |  | 55.4 |  |  | 31.6 |  |  | 56.2 |  |  | 32.2 |
AROs | 110.8 |  |  | 109.3 |  |  | 76.3 |  |  | 72.5 |  |  | 34.5 |  |  | 36.8 |
Derivatives | 28.0 |  |  | 45.3 |  |  | 15.1 |  |  | 21.8 |  |  | 12.9 |  |  | 23.5 |
Emission allowances | 23.6 |  |  | 25.5 |  |  | 23.6 |  |  | 25.5 |  |  | — |  |  | — |
Other | 100.4 |  |  | 96.6 |  |  | 51.5 |  |  | 55.3 |  |  | 48.9 |  |  | 41.3 |
|  | $1,737.3 |  |  |  | $1,666.7 |  |  |  | $1,279.0 |  |  |  | $1,231.6 |  |  |  | $458.3 |  |  |  | $435.1 |
At December 31, 2018, IPL and WPL had $77 million and $6 million, respectively, of regulatory assets that were not earning a return. IPL’s regulatory assets that were not earning a return consisted primarily of certain EGUs retired early, 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. 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. During 2018, Alliant Energy’s and IPL’s tax-related regulatory assets increased primarily due to property-related differences for qualifying repairs expenditures. Partially offsetting this increase was a decrease due to the impacts of Iowa tax reform as discussed in Note 12. Pension and other postretirement benefits costs - The IUB, PSCW and FERC have authorized IPL and WPL to record the previously unrecognized net actuarial gains and losses, and prior service costs and credits, as regulatory assets in lieu of accumulated other comprehensive loss on the balance sheets, as these amounts are expected to be recovered in future rates. 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 retail and wholesale customers, which are based upon pension and OPEB costs determined in accordance with GAAP and are calculated in accordance with IPL’s and WPL’s respective regulatory jurisdictions. Electric generating units retired early - IPL and WPL have retired various natural gas-fired and coal-fired EGUs and reclassified the remaining net book value of these EGUs from property, plant and equipment to regulatory assets on their respective balance sheets. Details regarding the recovery of the remaining net book value of these EGUs from IPL’s and WPL’s customers are as follows:
Entity |  | EGU |  | Retirement Date |  | Regulatory Asset Balance as of Dec. 31, 2018 |  | Recovery |  | Regulatory Approval
IPL |  | Sutherland Units 1 and 3 |  | June 2017 |  |  | $27.6 |  |  | Return of remaining net book value over 10 years |  | IUB and FERC
IPL |  | M.L. Kapp Unit 2 |  | June 2018 |  | 27.8 |  |  | Return of and return on remaining net book value over 10 years |  | Pending with FERC; to be addressed with IUB
WPL |  | Nelson Dewey Units 1 and 2 and Edgewater Unit 3 |  | December 2015 |  | 27.4 |  |  | Return of and return on remaining net book value over 10 years |  | PSCW and FERC
WPL |  | Edgewater Unit 4 |  | September 2018 |  | 28.8 |  |  | Return of and return on remaining net book value over 10 years |  | PSCW and pending with FERC
AROs - Alliant Energy, IPL and WPL believe it is probable that certain 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. 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. 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 and are authorized to recover these amounts from its retail customers over a 10-year period. Regulatory Liabilities - At December 31, regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Tax-related |  | $890.6 |  |  |  | $899.4 |  |  |  | $390.1 |  |  |  | $399.5 |  |  |  | $500.5 |  |  |  | $499.9 |
Cost of removal obligations | 401.2 |  |  | 410.0 |  |  | 273.3 |  |  | 274.5 |  |  | 127.9 |  |  | 135.5 |
Electric transmission cost recovery | 104.0 |  |  | 90.4 |  |  | 47.7 |  |  | 26.4 |  |  | 56.3 |  |  | 64.0 |
WPL earnings sharing mechanism | 25.4 |  |  | 8.0 |  |  | — |  |  | — |  |  | 25.4 |  |  | 8.0 |
Commodity cost recovery | 16.8 |  |  | 21.0 |  |  | 11.9 |  |  | 14.6 |  |  | 4.9 |  |  | 6.4 |
IPL’s tax benefit riders | 6.4 |  |  | 25.0 |  |  | 6.4 |  |  | 25.0 |  |  | — |  |  | — |
Other | 48.8 |  |  | 43.4 |  |  | 25.5 |  |  | 15.4 |  |  | 23.3 |  |  | 28.0 |
|  | $1,493.2 |  |  |  | $1,497.2 |  |  |  | $754.9 |  |  |  | $755.4 |  |  |  | $738.3 |  |  |  | $741.8 |
Regulatory liabilities related to cost of removal obligations, to the extent expensed through depreciation rates, reduce rate base. Tax-related regulatory liabilities related to excess deferred taxes also reduce revenue requirement calculations utilized in IPL’s and WPL’s respective rate proceedings. A significant portion of the remaining regulatory liabilities are not used to adjust revenue requirement calculations. Tax-related - Alliant Energy’s, IPL’s and WPL’s tax-related regulatory liabilities are primarily related to excess deferred tax benefits resulting from the remeasurement of accumulated deferred income taxes caused by Federal Tax Reform. A portion of these benefits related to accelerated depreciation are subject to tax normalization rules. These rules limit the rate at which these tax benefits are allowed to be passed on to customers. 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 amounts collected in rates for these future removal costs and reduce the regulatory liability for amounts spent on removal activities. Electric transmission cost recovery - Refer to Note 1(g) for details of IPL’s and WPL’s electric transmission cost recovery mechanisms. Starting in March 2018, amounts billed by transmission providers decreased due to the impacts from Federal Tax Reform. During 2018, Alliant Energy, IPL and WPL recorded the benefits associated with lower transmission expense as regulatory liabilities. In May 2018, IPL starting providing these benefits back to its retail electric customers utilizing the transmission recovery mechanism. WPL is deferring these benefits until a future rate proceeding. Offsetting WPL’s increase in regulatory liabilities was transmission expense amortizations as authorized in the WPL retail electric rate review (2017/2018 Test Period). WPL earnings sharing mechanism - Pursuant to PSCW orders, WPL must defer a portion of its earnings if its annual regulatory return on common equity exceeds certain levels during the related test periods. As of December 31, 2018, Alliant Energy and WPL deferred $20 million of WPL’s 2018 earnings related to this provision. The timing of the refund of these regulatory liabilities to customers will be determined in a future WPL regulatory proceeding. Commodity cost recovery - Refer to Note 1(g) for details of IPL’s and WPL’s commodity 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 2018, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities decreased by $19 million as follows (in millions):
Electric tax benefit rider credits |  | $17 |
Gas tax benefit rider credits | 2 |
|  | $19 |
Other - In January 2018, the IUB issued an order requiring IPL and other investor-owned utilities in Iowa to track all calculated differences since January 1, 2018 resulting from Federal Tax Reform, such that any over-collections can be refunded to its customers at a future date. Pursuant to IUB approval, the retail electric portion of IPL’s Federal Tax Reform benefits is currently being refunded to customers, beginning May 2018. In January 2018, the PSCW issued an order directing WPL and other investor-owned utilities in Wisconsin to defer the revenue requirement impacts resulting from Federal Tax Reform since its inception. Pursuant to PSCW approval, the retail electric and gas portions of WPL’s Federal Tax Reform benefits are currently being refunded to customers, beginning June 2018. In 2018, Alliant Energy, IPL, and WPL refunded benefits of $66 million, $25 million and $41 million, respectively, from the 2018 impact of Federal Tax Reform, which were recorded as a reduction in revenues. As of December 31, 2018, Alliant Energy’s, IPL’s and WPL’s remaining deferrals related to Federal Tax Reform were $20 million, $15 million and $5 million, respectively, which are included in “Other” in the regulatory liabilities table above. 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 and 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 September 2017, IPL reached a partial, non-unanimous settlement agreement with intervener groups 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 were effective May 1, 2018. 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 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 with implementation of final rates, 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 (2019/2020 Test Period) - In December 2018, the PSCW issued an order approving WPL’s proposed settlement for its retail electric and gas rate review covering the 2019/2020 Test Period, which was based on a stipulated agreement between WPL and intervener groups. Under the settlement, WPL retail electric and gas base rates will not change from current levels through the end of 2020. 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 order included utilization of amounts that WPL previously over-recovered from its customers for energy efficiency cost recovery and electric transmission cost recovery to reduce the requested base rate increase. The order also included provisions that required WPL to defer a portion of its earnings if its annual regulatory return on common equity exceeded certain levels in 2017 or 2018.
IPL [Member] |
Regulatory Matters [Line Items] |
Regulatory Matters | REGULATORY MATTERS Regulatory Assets - 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, 2018 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. At December 31, regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Tax-related |  | $820.6 |  |  |  | $778.2 |  |  |  | $783.1 |  |  |  | $750.5 |  |  |  | $37.5 |  |  |  | $27.7 |
Pension and OPEB costs | 542.3 |  |  | 548.0 |  |  | 274.0 |  |  | 274.4 |  |  | 268.3 |  |  | 273.6 |
EGUs retired early | 111.6 |  |  | 63.8 |  |  | 55.4 |  |  | 31.6 |  |  | 56.2 |  |  | 32.2 |
AROs | 110.8 |  |  | 109.3 |  |  | 76.3 |  |  | 72.5 |  |  | 34.5 |  |  | 36.8 |
Derivatives | 28.0 |  |  | 45.3 |  |  | 15.1 |  |  | 21.8 |  |  | 12.9 |  |  | 23.5 |
Emission allowances | 23.6 |  |  | 25.5 |  |  | 23.6 |  |  | 25.5 |  |  | — |  |  | — |
Other | 100.4 |  |  | 96.6 |  |  | 51.5 |  |  | 55.3 |  |  | 48.9 |  |  | 41.3 |
|  | $1,737.3 |  |  |  | $1,666.7 |  |  |  | $1,279.0 |  |  |  | $1,231.6 |  |  |  | $458.3 |  |  |  | $435.1 |
At December 31, 2018, IPL and WPL had $77 million and $6 million, respectively, of regulatory assets that were not earning a return. IPL’s regulatory assets that were not earning a return consisted primarily of certain EGUs retired early, 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. 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. During 2018, Alliant Energy’s and IPL’s tax-related regulatory assets increased primarily due to property-related differences for qualifying repairs expenditures. Partially offsetting this increase was a decrease due to the impacts of Iowa tax reform as discussed in Note 12. Pension and other postretirement benefits costs - The IUB, PSCW and FERC have authorized IPL and WPL to record the previously unrecognized net actuarial gains and losses, and prior service costs and credits, as regulatory assets in lieu of accumulated other comprehensive loss on the balance sheets, as these amounts are expected to be recovered in future rates. 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 retail and wholesale customers, which are based upon pension and OPEB costs determined in accordance with GAAP and are calculated in accordance with IPL’s and WPL’s respective regulatory jurisdictions. Electric generating units retired early - IPL and WPL have retired various natural gas-fired and coal-fired EGUs and reclassified the remaining net book value of these EGUs from property, plant and equipment to regulatory assets on their respective balance sheets. Details regarding the recovery of the remaining net book value of these EGUs from IPL’s and WPL’s customers are as follows:
Entity |  | EGU |  | Retirement Date |  | Regulatory Asset Balance as of Dec. 31, 2018 |  | Recovery |  | Regulatory Approval
IPL |  | Sutherland Units 1 and 3 |  | June 2017 |  |  | $27.6 |  |  | Return of remaining net book value over 10 years |  | IUB and FERC
IPL |  | M.L. Kapp Unit 2 |  | June 2018 |  | 27.8 |  |  | Return of and return on remaining net book value over 10 years |  | Pending with FERC; to be addressed with IUB
WPL |  | Nelson Dewey Units 1 and 2 and Edgewater Unit 3 |  | December 2015 |  | 27.4 |  |  | Return of and return on remaining net book value over 10 years |  | PSCW and FERC
WPL |  | Edgewater Unit 4 |  | September 2018 |  | 28.8 |  |  | Return of and return on remaining net book value over 10 years |  | PSCW and pending with FERC
AROs - Alliant Energy, IPL and WPL believe it is probable that certain 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. 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. 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 and are authorized to recover these amounts from its retail customers over a 10-year period. Regulatory Liabilities - At December 31, regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Tax-related |  | $890.6 |  |  |  | $899.4 |  |  |  | $390.1 |  |  |  | $399.5 |  |  |  | $500.5 |  |  |  | $499.9 |
Cost of removal obligations | 401.2 |  |  | 410.0 |  |  | 273.3 |  |  | 274.5 |  |  | 127.9 |  |  | 135.5 |
Electric transmission cost recovery | 104.0 |  |  | 90.4 |  |  | 47.7 |  |  | 26.4 |  |  | 56.3 |  |  | 64.0 |
WPL earnings sharing mechanism | 25.4 |  |  | 8.0 |  |  | — |  |  | — |  |  | 25.4 |  |  | 8.0 |
Commodity cost recovery | 16.8 |  |  | 21.0 |  |  | 11.9 |  |  | 14.6 |  |  | 4.9 |  |  | 6.4 |
IPL’s tax benefit riders | 6.4 |  |  | 25.0 |  |  | 6.4 |  |  | 25.0 |  |  | — |  |  | — |
Other | 48.8 |  |  | 43.4 |  |  | 25.5 |  |  | 15.4 |  |  | 23.3 |  |  | 28.0 |
|  | $1,493.2 |  |  |  | $1,497.2 |  |  |  | $754.9 |  |  |  | $755.4 |  |  |  | $738.3 |  |  |  | $741.8 |
Regulatory liabilities related to cost of removal obligations, to the extent expensed through depreciation rates, reduce rate base. Tax-related regulatory liabilities related to excess deferred taxes also reduce revenue requirement calculations utilized in IPL’s and WPL’s respective rate proceedings. A significant portion of the remaining regulatory liabilities are not used to adjust revenue requirement calculations. Tax-related - Alliant Energy’s, IPL’s and WPL’s tax-related regulatory liabilities are primarily related to excess deferred tax benefits resulting from the remeasurement of accumulated deferred income taxes caused by Federal Tax Reform. A portion of these benefits related to accelerated depreciation are subject to tax normalization rules. These rules limit the rate at which these tax benefits are allowed to be passed on to customers. 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 amounts collected in rates for these future removal costs and reduce the regulatory liability for amounts spent on removal activities. Electric transmission cost recovery - Refer to Note 1(g) for details of IPL’s and WPL’s electric transmission cost recovery mechanisms. Starting in March 2018, amounts billed by transmission providers decreased due to the impacts from Federal Tax Reform. During 2018, Alliant Energy, IPL and WPL recorded the benefits associated with lower transmission expense as regulatory liabilities. In May 2018, IPL starting providing these benefits back to its retail electric customers utilizing the transmission recovery mechanism. WPL is deferring these benefits until a future rate proceeding. Offsetting WPL’s increase in regulatory liabilities was transmission expense amortizations as authorized in the WPL retail electric rate review (2017/2018 Test Period). WPL earnings sharing mechanism - Pursuant to PSCW orders, WPL must defer a portion of its earnings if its annual regulatory return on common equity exceeds certain levels during the related test periods. As of December 31, 2018, Alliant Energy and WPL deferred $20 million of WPL’s 2018 earnings related to this provision. The timing of the refund of these regulatory liabilities to customers will be determined in a future WPL regulatory proceeding. Commodity cost recovery - Refer to Note 1(g) for details of IPL’s and WPL’s commodity 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 2018, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities decreased by $19 million as follows (in millions):
Electric tax benefit rider credits |  | $17 |
Gas tax benefit rider credits | 2 |
|  | $19 |
Other - In January 2018, the IUB issued an order requiring IPL and other investor-owned utilities in Iowa to track all calculated differences since January 1, 2018 resulting from Federal Tax Reform, such that any over-collections can be refunded to its customers at a future date. Pursuant to IUB approval, the retail electric portion of IPL’s Federal Tax Reform benefits is currently being refunded to customers, beginning May 2018. In January 2018, the PSCW issued an order directing WPL and other investor-owned utilities in Wisconsin to defer the revenue requirement impacts resulting from Federal Tax Reform since its inception. Pursuant to PSCW approval, the retail electric and gas portions of WPL’s Federal Tax Reform benefits are currently being refunded to customers, beginning June 2018. In 2018, Alliant Energy, IPL, and WPL refunded benefits of $66 million, $25 million and $41 million, respectively, from the 2018 impact of Federal Tax Reform, which were recorded as a reduction in revenues. As of December 31, 2018, Alliant Energy’s, IPL’s and WPL’s remaining deferrals related to Federal Tax Reform were $20 million, $15 million and $5 million, respectively, which are included in “Other” in the regulatory liabilities table above. 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 and 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 September 2017, IPL reached a partial, non-unanimous settlement agreement with intervener groups 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 were effective May 1, 2018. 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 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 with implementation of final rates, 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 (2019/2020 Test Period) - In December 2018, the PSCW issued an order approving WPL’s proposed settlement for its retail electric and gas rate review covering the 2019/2020 Test Period, which was based on a stipulated agreement between WPL and intervener groups. Under the settlement, WPL retail electric and gas base rates will not change from current levels through the end of 2020. 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 order included utilization of amounts that WPL previously over-recovered from its customers for energy efficiency cost recovery and electric transmission cost recovery to reduce the requested base rate increase. The order also included provisions that required WPL to defer a portion of its earnings if its annual regulatory return on common equity exceeded certain levels in 2017 or 2018.
WPL [Member] |
Regulatory Matters [Line Items] |
Regulatory Matters | REGULATORY MATTERS Regulatory Assets - 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, 2018 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. At December 31, regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Tax-related |  | $820.6 |  |  |  | $778.2 |  |  |  | $783.1 |  |  |  | $750.5 |  |  |  | $37.5 |  |  |  | $27.7 |
Pension and OPEB costs | 542.3 |  |  | 548.0 |  |  | 274.0 |  |  | 274.4 |  |  | 268.3 |  |  | 273.6 |
EGUs retired early | 111.6 |  |  | 63.8 |  |  | 55.4 |  |  | 31.6 |  |  | 56.2 |  |  | 32.2 |
AROs | 110.8 |  |  | 109.3 |  |  | 76.3 |  |  | 72.5 |  |  | 34.5 |  |  | 36.8 |
Derivatives | 28.0 |  |  | 45.3 |  |  | 15.1 |  |  | 21.8 |  |  | 12.9 |  |  | 23.5 |
Emission allowances | 23.6 |  |  | 25.5 |  |  | 23.6 |  |  | 25.5 |  |  | — |  |  | — |
Other | 100.4 |  |  | 96.6 |  |  | 51.5 |  |  | 55.3 |  |  | 48.9 |  |  | 41.3 |
|  | $1,737.3 |  |  |  | $1,666.7 |  |  |  | $1,279.0 |  |  |  | $1,231.6 |  |  |  | $458.3 |  |  |  | $435.1 |
At December 31, 2018, IPL and WPL had $77 million and $6 million, respectively, of regulatory assets that were not earning a return. IPL’s regulatory assets that were not earning a return consisted primarily of certain EGUs retired early, 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. 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. During 2018, Alliant Energy’s and IPL’s tax-related regulatory assets increased primarily due to property-related differences for qualifying repairs expenditures. Partially offsetting this increase was a decrease due to the impacts of Iowa tax reform as discussed in Note 12. Pension and other postretirement benefits costs - The IUB, PSCW and FERC have authorized IPL and WPL to record the previously unrecognized net actuarial gains and losses, and prior service costs and credits, as regulatory assets in lieu of accumulated other comprehensive loss on the balance sheets, as these amounts are expected to be recovered in future rates. 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 retail and wholesale customers, which are based upon pension and OPEB costs determined in accordance with GAAP and are calculated in accordance with IPL’s and WPL’s respective regulatory jurisdictions. Electric generating units retired early - IPL and WPL have retired various natural gas-fired and coal-fired EGUs and reclassified the remaining net book value of these EGUs from property, plant and equipment to regulatory assets on their respective balance sheets. Details regarding the recovery of the remaining net book value of these EGUs from IPL’s and WPL’s customers are as follows:
Entity |  | EGU |  | Retirement Date |  | Regulatory Asset Balance as of Dec. 31, 2018 |  | Recovery |  | Regulatory Approval
IPL |  | Sutherland Units 1 and 3 |  | June 2017 |  |  | $27.6 |  |  | Return of remaining net book value over 10 years |  | IUB and FERC
IPL |  | M.L. Kapp Unit 2 |  | June 2018 |  | 27.8 |  |  | Return of and return on remaining net book value over 10 years |  | Pending with FERC; to be addressed with IUB
WPL |  | Nelson Dewey Units 1 and 2 and Edgewater Unit 3 |  | December 2015 |  | 27.4 |  |  | Return of and return on remaining net book value over 10 years |  | PSCW and FERC
WPL |  | Edgewater Unit 4 |  | September 2018 |  | 28.8 |  |  | Return of and return on remaining net book value over 10 years |  | PSCW and pending with FERC
AROs - Alliant Energy, IPL and WPL believe it is probable that certain 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. 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. 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 and are authorized to recover these amounts from its retail customers over a 10-year period. Regulatory Liabilities - At December 31, regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Tax-related |  | $890.6 |  |  |  | $899.4 |  |  |  | $390.1 |  |  |  | $399.5 |  |  |  | $500.5 |  |  |  | $499.9 |
Cost of removal obligations | 401.2 |  |  | 410.0 |  |  | 273.3 |  |  | 274.5 |  |  | 127.9 |  |  | 135.5 |
Electric transmission cost recovery | 104.0 |  |  | 90.4 |  |  | 47.7 |  |  | 26.4 |  |  | 56.3 |  |  | 64.0 |
WPL earnings sharing mechanism | 25.4 |  |  | 8.0 |  |  | — |  |  | — |  |  | 25.4 |  |  | 8.0 |
Commodity cost recovery | 16.8 |  |  | 21.0 |  |  | 11.9 |  |  | 14.6 |  |  | 4.9 |  |  | 6.4 |
IPL’s tax benefit riders | 6.4 |  |  | 25.0 |  |  | 6.4 |  |  | 25.0 |  |  | — |  |  | — |
Other | 48.8 |  |  | 43.4 |  |  | 25.5 |  |  | 15.4 |  |  | 23.3 |  |  | 28.0 |
|  | $1,493.2 |  |  |  | $1,497.2 |  |  |  | $754.9 |  |  |  | $755.4 |  |  |  | $738.3 |  |  |  | $741.8 |
Regulatory liabilities related to cost of removal obligations, to the extent expensed through depreciation rates, reduce rate base. Tax-related regulatory liabilities related to excess deferred taxes also reduce revenue requirement calculations utilized in IPL’s and WPL’s respective rate proceedings. A significant portion of the remaining regulatory liabilities are not used to adjust revenue requirement calculations. Tax-related - Alliant Energy’s, IPL’s and WPL’s tax-related regulatory liabilities are primarily related to excess deferred tax benefits resulting from the remeasurement of accumulated deferred income taxes caused by Federal Tax Reform. A portion of these benefits related to accelerated depreciation are subject to tax normalization rules. These rules limit the rate at which these tax benefits are allowed to be passed on to customers. 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 amounts collected in rates for these future removal costs and reduce the regulatory liability for amounts spent on removal activities. Electric transmission cost recovery - Refer to Note 1(g) for details of IPL’s and WPL’s electric transmission cost recovery mechanisms. Starting in March 2018, amounts billed by transmission providers decreased due to the impacts from Federal Tax Reform. During 2018, Alliant Energy, IPL and WPL recorded the benefits associated with lower transmission expense as regulatory liabilities. In May 2018, IPL starting providing these benefits back to its retail electric customers utilizing the transmission recovery mechanism. WPL is deferring these benefits until a future rate proceeding. Offsetting WPL’s increase in regulatory liabilities was transmission expense amortizations as authorized in the WPL retail electric rate review (2017/2018 Test Period). WPL earnings sharing mechanism - Pursuant to PSCW orders, WPL must defer a portion of its earnings if its annual regulatory return on common equity exceeds certain levels during the related test periods. As of December 31, 2018, Alliant Energy and WPL deferred $20 million of WPL’s 2018 earnings related to this provision. The timing of the refund of these regulatory liabilities to customers will be determined in a future WPL regulatory proceeding. Commodity cost recovery - Refer to Note 1(g) for details of IPL’s and WPL’s commodity 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 2018, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities decreased by $19 million as follows (in millions):
Electric tax benefit rider credits |  | $17 |
Gas tax benefit rider credits | 2 |
|  | $19 |
Other - In January 2018, the IUB issued an order requiring IPL and other investor-owned utilities in Iowa to track all calculated differences since January 1, 2018 resulting from Federal Tax Reform, such that any over-collections can be refunded to its customers at a future date. Pursuant to IUB approval, the retail electric portion of IPL’s Federal Tax Reform benefits is currently being refunded to customers, beginning May 2018. In January 2018, the PSCW issued an order directing WPL and other investor-owned utilities in Wisconsin to defer the revenue requirement impacts resulting from Federal Tax Reform since its inception. Pursuant to PSCW approval, the retail electric and gas portions of WPL’s Federal Tax Reform benefits are currently being refunded to customers, beginning June 2018. In 2018, Alliant Energy, IPL, and WPL refunded benefits of $66 million, $25 million and $41 million, respectively, from the 2018 impact of Federal Tax Reform, which were recorded as a reduction in revenues. As of December 31, 2018, Alliant Energy’s, IPL’s and WPL’s remaining deferrals related to Federal Tax Reform were $20 million, $15 million and $5 million, respectively, which are included in “Other” in the regulatory liabilities table above. 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 and 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 September 2017, IPL reached a partial, non-unanimous settlement agreement with intervener groups 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 were effective May 1, 2018. 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 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 with implementation of final rates, 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 (2019/2020 Test Period) - In December 2018, the PSCW issued an order approving WPL’s proposed settlement for its retail electric and gas rate review covering the 2019/2020 Test Period, which was based on a stipulated agreement between WPL and intervener groups. Under the settlement, WPL retail electric and gas base rates will not change from current levels through the end of 2020. 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 order included utilization of amounts that WPL previously over-recovered from its customers for energy efficiency cost recovery and electric transmission cost recovery to reduce the requested base rate increase. The order also included provisions that required WPL to defer a portion of its earnings if its annual regulatory return on common equity exceeded certain levels in 2017 or 2018.

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