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Regulatory Matters (Narrative) (Details) - USD ($) $ in Millions | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2015 | Jun. 30, 2014 | Dec. 31, 2016 | Dec. 31, 2014
Regulatory Matters [Line Items] |  |  |  |
Decrease in regulatory liabilities | $ (40.0) | $ 45.2 |  |
Alliant Energy and IPL [Member] | Tax benefit riders [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Decrease in regulatory liabilities | 41.0 |  |  |
WPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Decrease in regulatory liabilities | (1.1) | $ 47.9 |  |
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] |  |  |  |
Under-collection of retail fuel-related costs |  |  |  | $ 33.0
Fuel-related costs | $ 28.0 |  |  |
Subsequent Event [Member] | 2016 Test Year Retail Electric Fuel Related [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Requested rate increase (decrease), amount |  |  | $ 15.0 |
Requested rate increase (decrease), percentage |  |  | 1.00% |
Subsequent Event [Member] | 2014 Test Year Retail Electric Fuel Related [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Approved rate increase (decrease), amount |  |  | $ 28.0 |
Approved rate increase (decrease), percentage |  |  | 3.00% |

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