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Regulatory Matters (Narrative) (Details) - WPL [Member] - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2015
2015 Test Year Retail Electric Fuel Related [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Annual bandwidth for fuel-related costs |  |  | 2.00%
Scenario, Forecast [Member] | 2017/2018 Test Period Retail Electric [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Requested rate increase (decrease), amount |  | $ 13 |
Requested rate increase (decrease), percentage |  | 1.00% |
Scenario, Forecast [Member] | 2017 Test Period Retail Electric Base Rate [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Requested rate increase (decrease), amount |  | $ 65 |
Scenario, Forecast [Member] | 2017 Test Year Retail Electric Fuel Related [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Requested rate increase (decrease), amount |  | (52) |
Scenario, Forecast [Member] | 2017/2018 Test Period Retail Gas [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Requested rate increase (decrease), amount |  | $ 9 |
Requested rate increase (decrease), percentage |  | 13.00% |
Scenario, Forecast [Member] | Subsequent Event [Member] | 2015 Test Year Retail Electric Fuel Related [Member] |  |  |
Regulatory Matters [Line Items] |  |  |
Authorized refund to customers, including interest | $ 10 |  |

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