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Related Parties (Amounts Billed Between Parties) (Details) - ATC [Member] - Equity Method Investee [Member] - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2019 | Mar. 31, 2018
ATC Billings To WPL [Member] |  |
Related Party Transactions [Line Items] |  |
Amounts billed between related parties | $ 27 | $ 27
WPL Billings To ATC [Member] |  |
Related Party Transactions [Line Items] |  |
Amounts billed between related parties | $ 4 | $ 2

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