Price Risk Management Price Risk Management (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Collateral, Master Netting Arrangements, Letters of Credit	$ 14		$ 14		$ 11
Net gain or (loss) recognized in the statement of income offset by regulatory accounting	34	$ 34	150	$ 30
Derivative, Net Liability Position, Aggregate Fair Value	291		291
Collateral Posted, Aggregate Fair Value	62		62
Letters of Credit Outstanding, Amount	51		51
Restricted Cash and Cash Equivalents, Current	11		11
Collateral cash requirement	278		278
Natural Gas [Member]
Derivative Instruments and Hedges, Liabilities	10		10		17
4911 Electric Services [Member]
Derivative Instruments and Hedges, Liabilities	118		118		55
Liabilities, Total [Member]
Derivative Instruments and Hedges, Liabilities	$ 128		$ 128		$ 72