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Regulatory Matters (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended | 36 Months Ended
--- | --- | ---
Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2016
Regulatory Matters [Line Items] |  |  |  |  |
Increase (decrease) in regulatory liabilities |  | $ (67.8) | $ 10.8 | $ (90.8) |
Electric revenues |  | 2,770.5 | 2,713.6 | 2,689.0 |
Regulatory liabilities |  | 737.7 | 821.2 |  |
IPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Regulatory assets not earning a return |  | $ 30.0 |  |  |
Period for simple average of actual costs incurred during Test Year |  | 2 years |  |  |
Increase (decrease) in regulatory liabilities |  | $ (75.5) | (18.9) | (82.3) |
Electric revenues |  | 1,503.8 | 1,493.3 | 1,491.8 |
Regulatory liabilities |  | 489.2 | 583.5 |  |
Billing credits to reduce retail electric customers' bills |  | 24.0 | 72.0 |  |
WPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Regulatory assets not earning a return |  | $ 12.0 |  |  |
Expenditure collection period after new rate implementation |  | 5 years |  |  |
Increase (decrease) in regulatory liabilities |  | $ 7.7 | 29.7 | (8.5) |
Electric revenues |  | 1,266.7 | 1,220.3 | 1,197.2 |
Regulatory liabilities |  | 248.5 | 237.7 |  |
WPL [Member] | 2013/2014 Test Period Retail Electric And Gas Final Order [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Authorized increase (decrease) in final rates, amount |  |  | (13.0) |  |
Alliant Energy and IPL [Member] | Revenue Requirement Adjustment [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Electric revenues |  | 14.0 | 15.0 | 24.0 |
Alliant Energy and WPL [Member] | Deferral [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Regulatory liabilities |  | 8.0 |  |  |
Scenario, Forecast [Member] | IPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Billing credits to reduce retail electric customers' bills | $ 9.0 |  |  |  | $ 105.0
Scenario, Forecast [Member] | Alliant Energy and IPL [Member] | Revenue Requirement Adjustment [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Electric revenues | 15.0 |  |  |  |
Electric [Member] | Alliant Energy and IPL [Member] | IPL's tax benefit riders [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Increase (decrease) in regulatory liabilities |  | (72.0) | (85.0) | (79.0) |
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 | (65.0) |  |  |  |
Gas [Member] | IPL's tax benefit riders [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Increase (decrease) in regulatory liabilities |  | (12.0) |  |  |
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) | $ (11.0) |
Nelson Dewey Units 1 and 2 and Edgewater Unit Three [Member] | WPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Expenditure collection period after new rate implementation |  | 10 years |  |  |
2014 Test Year Retail Electric Fuel Related [Member] | WPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Annual bandwidth for fuel-related costs |  | 2.00% |  |  |
2014 Test Year Retail Electric Fuel Related [Member] | Alliant Energy and WPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Retail electric fuel-related costs |  | $ 28.0 | 28.0 |  |
Under-collection of retail fuel related costs |  |  | 33.0 |  |
2015/2016 Test Period Retail Electric And Gas [Member] | WPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Authorized increase (decrease) in final rates, amount |  | (5.0) |  |  |
2016 Test Year Retail Electric Fuel Related [Member] | Scenario, Forecast [Member] | WPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Authorized increase (decrease) in final rates, amount | $ 7.0 |  |  |  |
Authorized increase (decrease) in final rates, percent | 1.00% |  |  |  |
Annual bandwidth for fuel-related costs | 2.00% |  |  |  |
2015 Test Year Retail Electric Fuel Related [Member] | WPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Authorized increase (decrease) in final rates, amount |  | $ 39.0 |  |  |
Authorized increase (decrease) in final rates, percent |  | 4.00% |  |  |
Annual bandwidth for fuel-related costs |  | 2.00% |  |  |
2015 Test Year Retail Electric Fuel Related [Member] | Alliant Energy and WPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Over-collection of retail fuel-related costs |  |  | $ 10.0 |  |
Fuel [Member] | WPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Retail electric fuel-related costs |  | $ 25.0 |  |  |
Sales [Member] | WPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Retail electric fuel-related costs |  | $ 14.0 |  |  |

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