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Notes Receivable (Narrative) (Details) - USD ($)
$ in Millions
Dec. 31, 2016
Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]    
EnerBank notes receivable held for sale $ 39 $ 16
EnerBank [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Unearned income 84 82
Delinquent loans 11 8
Loans modified as troubled debt restructurings 1 1
Receivables Held For Sale [Member] | EnerBank [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Unearned income $ 8 $ 3