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Related Parties (Related Amounts Billed Between Parties) (Details) - WPL [Member] - ATC LLC [Member] - Equity Method Investment [Member] - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
ATC Billings To WPL [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Amounts billed between related parties | $ 109 | $ 106 | $ 105
WPL Billings To ATC [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Amounts billed between related parties | $ 13 | $ 11 | $ 10

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