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Debt (Narrative) (Details) - USD ($) $ in Millions | 1 Months Ended
--- | ---
Oct. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Apr. 30, 2018
Term Loan Credit Agreement [Member] | Short-term loan credit agreement through 2018 [Member] | Alliant Energy Finance LLC [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Retirement of debt |  |  | $ 95 |
Term Loan Credit Agreement [Member] | Term loan credit agreement through April 2020 [Member] | Alliant Energy Finance LLC [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Proceeds from issuance of long-term debt |  |  |  | $ 300
Interest rate, percent |  | 2.80% |  |
Term Loan Credit Agreement [Member] | Term loan credit agreement through 2018 [Member] | Alliant Energy Finance LLC [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Retirement of debt |  |  | 500 |
Senior Notes [Member] | 3.75% senior notes due 2023 [Member] | Alliant Energy Finance LLC [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Proceeds from issuance of long-term debt |  |  | 400 |
Interest rate, percent |  | 3.75% |  |
Senior Notes [Member] | 4.25% senior notes due 2028 [Member] | Alliant Energy Finance LLC [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Proceeds from issuance of long-term debt |  |  | $ 300 |
Interest rate, percent |  | 4.25% |  |
Senior Debentures [Member] | 4.1% senior debentures due 2028 [Member] | IPL [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Proceeds from issuance of long-term debt |  | $ 500 |  |
Interest rate, percent |  | 4.10% |  |
Senior Debentures [Member] | 5.875% senior debentures due 2018 [Member] | IPL [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Retirement of debt |  | $ 100 |  |
Interest rate, percent |  | 5.875% |  |
Senior Debentures [Member] | 7.25% senior debentures due 2018 [Member] | IPL [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Interest rate, percent |  | 7.25% |  |
Senior Debentures [Member] | Subsequent Event [Member] | 7.25% senior debentures due 2018 [Member] | IPL [Member] |  |  |  |
Debt [Line Items] |  |  |  |
Retirement of debt | $ 250 |  |  |

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