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Debt (Narrative) (Details) - USD ($) $ in Millions | 1 Months Ended | 9 Months Ended
--- | --- | ---
Jun. 30, 2020 | Apr. 30, 2020 | Mar. 31, 2020 | Sep. 30, 2020 | Sep. 30, 2019
Debt [Line Items] |  |  |  |  |
Payments to retire long-term debt |  |  |  | $ 654 | $ 253
IPL [Member] |  |  |  |  |
Debt [Line Items] |  |  |  |  |
Payments to retire long-term debt |  |  |  | 200 | 0
WPL [Member] |  |  |  |  |
Debt [Line Items] |  |  |  |  |
Payments to retire long-term debt |  |  |  | $ 150 | $ 250
Term Loan Credit Agreement [Member] | Term Loan Credit Agreement Through March 2022 [Member] | Alliant Energy Finance, LLC [Member] |  |  |  |  |
Debt [Line Items] |  |  |  |  |
Proceeds from issuance of long-term debt |  |  | $ 300 |  |
Interest rate, percent |  |  |  | 1.00% |
Term Loan Credit Agreement [Member] | Term Loan Credit Agreement Through April 2020 [Member] | Alliant Energy Finance, LLC [Member] |  |  |  |  |
Debt [Line Items] |  |  |  |  |
Payments to retire long-term debt |  |  | $ 300 |  |
Debentures [Member] | 3.65% debenture, due 2050 [Member] | WPL [Member] |  |  |  |  |
Debt [Line Items] |  |  |  |  |
Proceeds from issuance of long-term debt |  | $ 350 |  |  |
Interest rate, percent |  |  |  | 3.65% |
Debentures [Member] | 4.6% debenture, due 2020 [Member] | WPL [Member] |  |  |  |  |
Debt [Line Items] |  |  |  |  |
Interest rate, percent | 4.60% |  |  |  |
Payments to retire long-term debt | $ 150 |  |  |  |
Senior Debentures [Member] | 2.3% senior debenture, due 2030 [Member] | IPL [Member] |  |  |  |  |
Debt [Line Items] |  |  |  |  |
Proceeds from issuance of long-term debt | $ 400 |  |  |  |
Interest rate, percent |  |  |  | 2.30% |
Senior Debentures [Member] | 3.65% senior debenture, due 2020 [Member] | IPL [Member] |  |  |  |  |
Debt [Line Items] |  |  |  |  |
Interest rate, percent | 3.65% |  |  |  |
Payments to retire long-term debt | $ 200 |  |  |  |

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