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v3.25.3

Debt (Narrative) (Details) | 9 Months Ended |  |
--- | --- | --- | ---
Sep. 30, 2025 USD ($) Integer $ / shares shares | Sep. 30, 2024 USD ($) | Mar. 31, 2026 USD ($) | Dec. 31, 2024 USD ($)
Debt [Line Items] |  |  |  |
Line of credit facility, current borrowing capacity | $ 1,300,000,000 |  |  |
Payments to retire long-term debt | 300,000,000 | $ 305,000,000 |  |
Long-term debt, fair value | 11,504,000,000 |  |  | $ 9,577,000,000
Parent Company [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Line of credit facility, current borrowing capacity | 550,000,000 |  |  |
IPL [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Line of credit facility, current borrowing capacity | 350,000,000 |  |  |
Payments to retire long-term debt | 300,000,000 | $ 0 |  |
Long-term debt, fair value | 4,465,000,000 |  |  | 3,736,000,000
WPL [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Line of credit facility, current borrowing capacity | 400,000,000 |  |  |
Long-term debt, fair value | 3,287,000,000 |  |  | $ 3,170,000,000
Term Loan Credit Agreement [Member] | Term loan credit agreement through March 2026 [Member] | Alliant Energy Finance LLC [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Long-term debt | $ 300,000,000 |  |  |
Interest rate, percent | 5.00% |  |  |
Term Loan Credit Agreement [Member] | Term loan credit agreement through March 2026 [Member] | Alliant Energy Finance LLC [Member] | Forecast [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Long-term debt |  |  | $ 100,000,000 |
Term Loan Credit Agreement [Member] | Term loan credit agreement through March 2025 [Member] | Alliant Energy Finance LLC [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Payments to retire long-term debt | $ 300,000,000 |  |  |
Senior Debentures [Member] | 5.6% senior debentures, due 2035 [Member] | IPL [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Long-term debt | $ 600,000,000 |  |  |
Interest rate, percent | 5.60% |  |  |
Senior Debentures [Member] | 5.5% senior debentures, due 2025 [Member] | IPL [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Interest rate, percent | 5.50% |  |  |
Payments to retire long-term debt | $ 50,000,000 |  |  |
Senior Debentures [Member] | 3.4% senior debentures, due 2025 [Member] | IPL [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Interest rate, percent | 3.40% |  |  |
Payments to retire long-term debt | $ 250,000,000 |  |  |
Senior Debentures [Member] | 5.6% Senior Debentures Due 2055 [Member] | IPL [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Long-term debt | $ 300,000,000 |  |  |
Interest rate, percent | 5.60% |  |  |
Junior Subordinated Debt [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Interest rate reset period, years | Integer | 5 |  |  |
Junior Subordinated Debt [Member] | 5.75% Fixed-to-Fixed Reset Rate Junior Subordinated Debt Due 2056 [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Long-term debt | $ 725,000,000 |  |  |
Interest rate, percent | 5.75% |  |  |
Minimum interest rate, percent | 5.75% |  |  |
Junior Subordinated Debt [Member] | 2.077% Spread on Fixed-to-Fixed Reset Rate Junior Subordinated Debt Due 2056 [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Spread, interest rate | 2.077% |  |  |
Senior Notes [Member] | 1.4% Senior Notes Due 2026 [Member] | Alliant Energy Finance LLC [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Long-term debt | $ 200,000,000 |  |  |
Interest rate, percent | 1.40% |  |  |
Convertible Senior Notes [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Trading days | Integer | 20 |  |  |
Trading days consecutive | Integer | 30 |  |  |
Convertible Senior Notes [Member] | 3.875% convertible senior notes, due 2026 [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Long-term debt | $ 575,000,000 |  |  |
Interest rate, percent | 3.875% |  |  |
Long-term debt, carrying value | $ 574,000,000 |  |  |
Unamortized debt issuance costs | 1,000,000 |  |  |
Long-term debt, fair value | $ 619,000,000 |  |  |
Shares included in diluted earnings per share (in shares) | shares | 27,905 |  |  |
Convertible Senior Notes [Member] | 3.25% convertible senior notes, due 2028 [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Long-term debt | $ 575,000,000 |  |  |
Interest rate, percent | 3.25% |  |  |
Trading days | Integer | 5 |  |  |
Trading days consecutive | Integer | 10 |  |  |
Principal amount of Notes conversion rate applied to | $ 1,000 |  |  |
Initial conversion ratio (in shares) | 13.1773 |  |  |
Initial conversion price (in dollars per share) | $ / shares | $ 75.89 |  |  |
Fundamental change repurchase conversion price, percent | 100.00% |  |  |
Long-term debt, carrying value | $ 569,000,000 |  |  |
Unamortized debt issuance costs | 6,000,000 |  |  |
Long-term debt, fair value | $ 593,000,000 |  |  |
Shares included in diluted earnings per share (in shares) | shares | 0 |  |  |
Convertible Senior Notes [Member] | 3.25% convertible senior notes, due 2028 [Member] | Maximum [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Conversion price, percent | 130.00% |  |  |
Threshold, product of last reported sale price of common stock and conversion rate, percent | 98.00% |  |  |

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