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Notes Receivable (Narrative) (Details) - USD ($)
$ in Millions
1 Months Ended 9 Months Ended
Mar. 31, 2017
Sep. 30, 2017
Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Sale of notes receivable   $ 19  
EnerBank notes receivable held for sale   31 $ 39
EnerBank [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Sale of notes receivable $ 19    
Unearned income   83 84
Delinquent loans   12 11
Loans modified as troubled debt restructurings   1 1
Receivables Held For Sale [Member] | EnerBank [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
EnerBank notes receivable held for sale   31 39
Unearned income   $ 6 $ 8