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Related Parties (Net Intercompany Payables) (Details) - Subsidiary of Common Parent [Member] - Corporate Services [Member] - USD ($)
$ in Millions
Sep. 30, 2020
Dec. 31, 2019
IPL [Member]    
Related Party Transactions [Line Items]    
Net amounts owed $ 119 $ 112
WPL [Member]    
Related Party Transactions [Line Items]    
Net amounts owed $ 77 $ 85