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Regulatory Matters (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2016 | Dec. 31, 2014 | Sep. 30, 2015
2015 Test Year Retail Electric Fuel Related [Member] | WPL [Member] |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Annual bandwidth for fuel-related costs |  | 2.00% |
2015 Test Year Retail Electric Fuel Related [Member] | Alliant Energy and WPL [Member] |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Fuel-related costs |  |  | $ 5
2014 Test Year Retail Electric Fuel Related [Member] | WPL [Member] |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Annual bandwidth for fuel-related costs |  | 2.00% |
Under-collection of retail fuel-related costs |  | $ 33 |
2014 Test Year Retail Electric Fuel Related [Member] | Alliant Energy and WPL [Member] |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Fuel-related costs |  |  | $ 28
Scenario, Forecast [Member] | 2016 Test Year Retail Electric Fuel Related [Member] | WPL [Member] |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Requested rate increase (decrease), amount | $ 15 |  |
Requested rate increase (decrease), percentage | 1.00% |  |
Scenario, Forecast [Member] | 2014 Test Year Retail Electric Fuel Related [Member] | WPL [Member] |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Approved rate increase (decrease), amount | $ 28 |  |
Approved rate increase (decrease), percentage | 3.00% |  |

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