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Regulatory Matters (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Regulatory Matters [Line Items] |  |  |  |
Increase (decrease) in regulatory liabilities |  | $ (63.0) | $ (67.8) | $ 10.8
Regulatory liabilities |  | 681.0 | 737.7 |
IPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Regulatory assets not earning a return |  | $ 44.0 |  |
Period for simple average of actual costs incurred during Test Year |  | 2 years |  |
Increase (decrease) in regulatory liabilities |  | $ (67.3) | (75.5) | (18.9)
Regulatory liabilities |  | 430.8 | 489.2 |
WPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Regulatory assets not earning a return |  | 9.0 |  |
Regulatory liabilities |  | 250.2 | 248.5 |
Electric [Member] | Alliant Energy and IPL [Member] | IPL's tax benefit riders [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Increase (decrease) in regulatory liabilities |  | (64.0) | (72.0) | (85.0)
Electric [Member] | Scenario, Forecast [Member] | Alliant Energy and IPL [Member] | IPL's tax benefit riders [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Increase (decrease) in regulatory liabilities | $ (68.0) |  |  |
Gas [Member] | Alliant Energy and IPL [Member] | IPL's tax benefit riders [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Increase (decrease) in regulatory liabilities |  | $ (12.0) | $ (12.0) | $ (12.0)
Gas [Member] | Scenario, Forecast [Member] | Alliant Energy and IPL [Member] | IPL's tax benefit riders [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Increase (decrease) in regulatory liabilities | (7.0) |  |  |
Nelson Dewey Units 1 and 2 and Edgewater Unit 3 [Member] | WPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Expenditure collection period after new rate implementation |  | 10 years |  |
2017/2018 Test Period Retail Electric [Member] | Scenario, Forecast [Member] | WPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Authorized increase (decrease) in final rates, amount | (9.0) |  |  |
2017/2018 Test Period Retail Gas [Member] | Scenario, Forecast [Member] | WPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Authorized increase (decrease) in final rates, amount | $ (9.0) |  |  |
2015/2016 Test Period Retail Electric And Gas [Member] | WPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Authorized increase (decrease) in final rates, amount |  | $ 5.0 |  |
2015/2016 Test Period Retail Electric And Gas [Member] | Alliant Energy and WPL [Member] | Deferral [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Regulatory liabilities |  | 6.0 |  |
2013/2014 Test Period Retail Electric And Gas [Member] | Alliant Energy and WPL [Member] | Deferral [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Regulatory liabilities |  | 6.0 |  |
2016 Test Year Retail Electric Fuel Related [Member] | WPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Authorized increase (decrease) in final rates, amount |  | $ 7.0 |  |
Annual bandwidth for fuel-related costs |  | 2.00% |  |

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