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Related Parties (Related Amounts Billed Between Parties) (Details) - WPL [Member] - ATC [Member] - Equity Method Investment [Member] - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
ATC Billings To WPL [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Amounts billed between related parties | $ 106 | $ 105 | $ 110
WPL Billings To ATC [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Amounts billed between related parties | $ 11 | $ 10 | $ 13

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