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Regulatory Matters (Retail Fuel-related Rate Filings) (Details) - WPL [Member] - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
2016 Test Year Retail Electric Fuel Related [Member] |  |  |
Regulatory Liabilities [Line Items] |  |  |
Authorized increase (decrease) in final rates, amount | $ 7 |  |
Authorized increase (decrease) in final rates, percent | 1.00% |  |
Retail electric fuel-related costs | $ 9 |  |
2015 Test Year Retail Electric Fuel Related [Member] |  |  |
Regulatory Liabilities [Line Items] |  |  |
Authorized increase (decrease) in final rates, amount |  | $ 39 |
Authorized increase (decrease) in final rates, percent |  | 4.00% |
Refund of over-collected fuel-related costs | 10 |  |
2014 Test Year Retail Electric Fuel Related [Member] |  |  |
Regulatory Liabilities [Line Items] |  |  |
Authorized increase (decrease) in final rates, amount |  |  | $ 19
Authorized increase (decrease) in final rates, percent |  |  | 2.00%
Deferral [Member] | 2014 Test Year Retail Electric Fuel Related [Member] |  |  |
Regulatory Liabilities [Line Items] |  |  |
Authorized increase (decrease) in final rates, amount | $ (28) |  |

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