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Debt (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2024 | Mar. 31, 2023 | Mar. 31, 2025 | Dec. 31, 2023
Debt [Line Items] |  |  |  |
Line of credit facility, current borrowing capacity | $ 1,000 |  |  |
Payments to retire long-term debt | 300 | $ 0 |  |
Long-term debt, fair value | 9,051 |  |  | $ 8,677
Parent Company [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Line of credit facility, current borrowing capacity | 500 |  |  |
IPL [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Line of credit facility, current borrowing capacity | 200 |  |  |
Long-term debt, fair value | 3,607 |  |  | 3,664
WPL [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Line of credit facility, current borrowing capacity | 300 |  |  |
Long-term debt, fair value | 3,176 |  |  | $ 2,933
Term Loan Credit Agreement [Member] | Term loan credit agreement through March 2025 [Member] | Alliant Energy Finance LLC [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Long-term debt | $ 300 |  |  |
Interest rate, percent | 6.00% |  |  |
Term Loan Credit Agreement [Member] | Term loan credit agreement through March 2025 [Member] | Alliant Energy Finance LLC [Member] | Forecast [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Long-term debt |  |  | $ 100 |
Term Loan Credit Agreement [Member] | Term loan credit agreement through March 2024 [Member] | Alliant Energy Finance LLC [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Payments to retire long-term debt | $ 300 |  |  |
Debentures [Member] | 5.375% debenture, due 2034 [Member] | WPL [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Long-term debt | $ 300 |  |  |
Interest rate, percent | 5.375% |  |  |
Convertible Senior Notes [Member] | 3.875% convertible senior notes, due 2026 [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Long-term debt, carrying value | $ 569 |  |  |
Unamortized debt issuance costs | 6 |  |  |
Long-term debt, fair value | $ 567 |  |  |
Shares included in diluted earnings per share (in shares) | 0 |  |  |

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