Notes Receivable (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2015	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Sale of notes receivable	$ 50	$ 48
EnerBank [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Sale of notes receivable	52
Unearned income	84		$ 84
Delinquent loans	14		11
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