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Regulatory Matters (Tables) | 12 Months Ended
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Dec. 31, 2024
Regulatory Assets [Line Items] |
Schedule of Regulatory Assets | At December 31, regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2024 |  | 2023 |  | 2024 |  | 2023 |  | 2024 |  | 2023
Tax-related | $989 |  |  | $934 |  |  | $870 |  |  | $831 |  |  | $119 |  |  | $103 |
AROs | 401 |  |  | 194 |  |  | 281 |  |  | 160 |  |  | 120 |  |  | 34 |
Pension and OPEB costs | 315 |  |  | 347 |  |  | 157 |  |  | 171 |  |  | 158 |  |  | 176 |
Assets retired early | 180 |  |  | 273 |  |  | 168 |  |  | 259 |  |  | 12 |  |  | 14 |
Commodity cost recovery | 68 |  |  | 120 |  |  | 2 |  |  | 12 |  |  | 66 |  |  | 108 |
Derivatives | 60 |  |  | 102 |  |  | 15 |  |  | 34 |  |  | 45 |  |  | 68 |
Non-service pension and OPEB costs | 51 |  |  | 46 |  |  | 19 |  |  | 18 |  |  | 32 |  |  | 28 |
WPL’s Western Wisconsin gas distribution expansion investments | 42 |  |  | 44 |  |  | — |  |  | — |  |  | 42 |  |  | 44 |
IPL’s DAEC PPA amendment | 18 |  |  | 42 |  |  | 18 |  |  | 42 |  |  | — |  |  | — |
Other | 150 |  |  | 159 |  |  | 56 |  |  | 50 |  |  | 94 |  |  | 109 |
| $2,274 |  |  | $2,261 |  |  | $1,586 |  |  | $1,577 |  |  | $688 |  |  | $684 |
Assets Retired Early | Details regarding the recovery of the remaining net book value of these assets from IPL’s and WPL’s customers are as follows (dollars in millions):
Entity |  | Asset |  | Retirement Date |  | Regulatory Asset Balance as of Dec. 31, 2024 |  | Recovery |  | Regulatory Approval
IPL |  | Lansing |  | 2023 |  | $131 |  | Return of (IUC and FERC) and return on (FERC) remaining net book value through 2037 (a) |  | IUC and FERC (b)
IPL |  | Analog electric meters |  | 2019 |  | 15 |  | Return of remaining net book value through 2028 |  | IUC and FERC
IPL |  | Sutherland Units 1 and 3 |  | 2017 |  | 15 |  | Return of and return on remaining net book value through 2027 (a) |  | IUC and FERC
IPL |  | M.L. Kapp Unit 2 |  | 2018 |  | 7 |  | Return of and return on remaining net book value through 2029 (a) |  | IUC and FERC
WPL |  | Edgewater Unit 4 |  | 2018 |  | 12 |  | Return of and return on remaining net book value through 2028 |  | PSCW and FERC
(a)The remaining regulatory asset balances include differences between expected and actual cost of removal obligations. (b)IPL was previously allowed a full recovery of and a full return on the Lansing Generation Station from both its retail and wholesale customers. The IUC’s September 2024 order for IPL’s retail electric rate review for the October 2024 through September 2025 forward-looking Test Period includes a return of the remaining net book value of Lansing, but does not include a return on the remaining net book value of Lansing, effective October 1, 2024. As a result, the return on the remaining net book value is no longer recoverable from IPL’s retail electric customers, and a pre-tax non-cash charge of $60 million was recorded to “Asset valuation charge for IPL’s Lansing Generating Station” in Alliant Energy’s and IPL’s income statements in 2024, with a corresponding decrease in Alliant Energy’s and IPL’s assets retired early regulatory assets.
Schedule of Regulatory Liabilities | At December 31, regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2024 |  | 2023 |  | 2024 |  | 2023 |  | 2024 |  | 2023
Tax-related | $582 |  | $566 |  | $286 |  | $299 |  | $296 |  | $267
Cost of removal obligations | 347 |  | 366 |  | 205 |  | 242 |  | 142 |  | 124
Derivatives | 53 |  | 65 |  | 29 |  | 34 |  | 24 |  | 31
Commodity cost recovery | 17 |  | 48 |  | 12 |  | 13 |  | 5 |  | 35
Other | 29 |  | 85 |  | 14 |  | 56 |  | 15 |  | 29
| $1,028 |  | $1,130 |  | $546 |  | $644 |  | $482 |  | $486
IPL [Member] |
Regulatory Assets [Line Items] |
Schedule of Regulatory Assets | At December 31, regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2024 |  | 2023 |  | 2024 |  | 2023 |  | 2024 |  | 2023
Tax-related | $989 |  |  | $934 |  |  | $870 |  |  | $831 |  |  | $119 |  |  | $103 |
AROs | 401 |  |  | 194 |  |  | 281 |  |  | 160 |  |  | 120 |  |  | 34 |
Pension and OPEB costs | 315 |  |  | 347 |  |  | 157 |  |  | 171 |  |  | 158 |  |  | 176 |
Assets retired early | 180 |  |  | 273 |  |  | 168 |  |  | 259 |  |  | 12 |  |  | 14 |
Commodity cost recovery | 68 |  |  | 120 |  |  | 2 |  |  | 12 |  |  | 66 |  |  | 108 |
Derivatives | 60 |  |  | 102 |  |  | 15 |  |  | 34 |  |  | 45 |  |  | 68 |
Non-service pension and OPEB costs | 51 |  |  | 46 |  |  | 19 |  |  | 18 |  |  | 32 |  |  | 28 |
WPL’s Western Wisconsin gas distribution expansion investments | 42 |  |  | 44 |  |  | — |  |  | — |  |  | 42 |  |  | 44 |
IPL’s DAEC PPA amendment | 18 |  |  | 42 |  |  | 18 |  |  | 42 |  |  | — |  |  | — |
Other | 150 |  |  | 159 |  |  | 56 |  |  | 50 |  |  | 94 |  |  | 109 |
| $2,274 |  |  | $2,261 |  |  | $1,586 |  |  | $1,577 |  |  | $688 |  |  | $684 |
Assets Retired Early | Details regarding the recovery of the remaining net book value of these assets from IPL’s and WPL’s customers are as follows (dollars in millions):
Entity |  | Asset |  | Retirement Date |  | Regulatory Asset Balance as of Dec. 31, 2024 |  | Recovery |  | Regulatory Approval
IPL |  | Lansing |  | 2023 |  | $131 |  | Return of (IUC and FERC) and return on (FERC) remaining net book value through 2037 (a) |  | IUC and FERC (b)
IPL |  | Analog electric meters |  | 2019 |  | 15 |  | Return of remaining net book value through 2028 |  | IUC and FERC
IPL |  | Sutherland Units 1 and 3 |  | 2017 |  | 15 |  | Return of and return on remaining net book value through 2027 (a) |  | IUC and FERC
IPL |  | M.L. Kapp Unit 2 |  | 2018 |  | 7 |  | Return of and return on remaining net book value through 2029 (a) |  | IUC and FERC
WPL |  | Edgewater Unit 4 |  | 2018 |  | 12 |  | Return of and return on remaining net book value through 2028 |  | PSCW and FERC
(a)The remaining regulatory asset balances include differences between expected and actual cost of removal obligations. (b)IPL was previously allowed a full recovery of and a full return on the Lansing Generation Station from both its retail and wholesale customers. The IUC’s September 2024 order for IPL’s retail electric rate review for the October 2024 through September 2025 forward-looking Test Period includes a return of the remaining net book value of Lansing, but does not include a return on the remaining net book value of Lansing, effective October 1, 2024. As a result, the return on the remaining net book value is no longer recoverable from IPL’s retail electric customers, and a pre-tax non-cash charge of $60 million was recorded to “Asset valuation charge for IPL’s Lansing Generating Station” in Alliant Energy’s and IPL’s income statements in 2024, with a corresponding decrease in Alliant Energy’s and IPL’s assets retired early regulatory assets.
Schedule of Regulatory Liabilities | At December 31, regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2024 |  | 2023 |  | 2024 |  | 2023 |  | 2024 |  | 2023
Tax-related | $582 |  | $566 |  | $286 |  | $299 |  | $296 |  | $267
Cost of removal obligations | 347 |  | 366 |  | 205 |  | 242 |  | 142 |  | 124
Derivatives | 53 |  | 65 |  | 29 |  | 34 |  | 24 |  | 31
Commodity cost recovery | 17 |  | 48 |  | 12 |  | 13 |  | 5 |  | 35
Other | 29 |  | 85 |  | 14 |  | 56 |  | 15 |  | 29
| $1,028 |  | $1,130 |  | $546 |  | $644 |  | $482 |  | $486
WPL [Member] |
Regulatory Assets [Line Items] |
Schedule of Regulatory Assets | At December 31, regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2024 |  | 2023 |  | 2024 |  | 2023 |  | 2024 |  | 2023
Tax-related | $989 |  |  | $934 |  |  | $870 |  |  | $831 |  |  | $119 |  |  | $103 |
AROs | 401 |  |  | 194 |  |  | 281 |  |  | 160 |  |  | 120 |  |  | 34 |
Pension and OPEB costs | 315 |  |  | 347 |  |  | 157 |  |  | 171 |  |  | 158 |  |  | 176 |
Assets retired early | 180 |  |  | 273 |  |  | 168 |  |  | 259 |  |  | 12 |  |  | 14 |
Commodity cost recovery | 68 |  |  | 120 |  |  | 2 |  |  | 12 |  |  | 66 |  |  | 108 |
Derivatives | 60 |  |  | 102 |  |  | 15 |  |  | 34 |  |  | 45 |  |  | 68 |
Non-service pension and OPEB costs | 51 |  |  | 46 |  |  | 19 |  |  | 18 |  |  | 32 |  |  | 28 |
WPL’s Western Wisconsin gas distribution expansion investments | 42 |  |  | 44 |  |  | — |  |  | — |  |  | 42 |  |  | 44 |
IPL’s DAEC PPA amendment | 18 |  |  | 42 |  |  | 18 |  |  | 42 |  |  | — |  |  | — |
Other | 150 |  |  | 159 |  |  | 56 |  |  | 50 |  |  | 94 |  |  | 109 |
| $2,274 |  |  | $2,261 |  |  | $1,586 |  |  | $1,577 |  |  | $688 |  |  | $684 |
Assets Retired Early | Details regarding the recovery of the remaining net book value of these assets from IPL’s and WPL’s customers are as follows (dollars in millions):
Entity |  | Asset |  | Retirement Date |  | Regulatory Asset Balance as of Dec. 31, 2024 |  | Recovery |  | Regulatory Approval
IPL |  | Lansing |  | 2023 |  | $131 |  | Return of (IUC and FERC) and return on (FERC) remaining net book value through 2037 (a) |  | IUC and FERC (b)
IPL |  | Analog electric meters |  | 2019 |  | 15 |  | Return of remaining net book value through 2028 |  | IUC and FERC
IPL |  | Sutherland Units 1 and 3 |  | 2017 |  | 15 |  | Return of and return on remaining net book value through 2027 (a) |  | IUC and FERC
IPL |  | M.L. Kapp Unit 2 |  | 2018 |  | 7 |  | Return of and return on remaining net book value through 2029 (a) |  | IUC and FERC
WPL |  | Edgewater Unit 4 |  | 2018 |  | 12 |  | Return of and return on remaining net book value through 2028 |  | PSCW and FERC
(a)The remaining regulatory asset balances include differences between expected and actual cost of removal obligations. (b)IPL was previously allowed a full recovery of and a full return on the Lansing Generation Station from both its retail and wholesale customers. The IUC’s September 2024 order for IPL’s retail electric rate review for the October 2024 through September 2025 forward-looking Test Period includes a return of the remaining net book value of Lansing, but does not include a return on the remaining net book value of Lansing, effective October 1, 2024. As a result, the return on the remaining net book value is no longer recoverable from IPL’s retail electric customers, and a pre-tax non-cash charge of $60 million was recorded to “Asset valuation charge for IPL’s Lansing Generating Station” in Alliant Energy’s and IPL’s income statements in 2024, with a corresponding decrease in Alliant Energy’s and IPL’s assets retired early regulatory assets.
Schedule of Regulatory Liabilities | At December 31, regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2024 |  | 2023 |  | 2024 |  | 2023 |  | 2024 |  | 2023
Tax-related | $582 |  | $566 |  | $286 |  | $299 |  | $296 |  | $267
Cost of removal obligations | 347 |  | 366 |  | 205 |  | 242 |  | 142 |  | 124
Derivatives | 53 |  | 65 |  | 29 |  | 34 |  | 24 |  | 31
Commodity cost recovery | 17 |  | 48 |  | 12 |  | 13 |  | 5 |  | 35
Other | 29 |  | 85 |  | 14 |  | 56 |  | 15 |  | 29
| $1,028 |  | $1,130 |  | $546 |  | $644 |  | $482 |  | $486

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