Price Risk Management Price Risk Management (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Sep. 30, 2014	Dec. 31, 2015
Collateral, Master Netting Arrangements, Letters of Credit	$ 14			$ 14
Net gain or (loss) recognized in the statement of income offset by regulatory accounting	34	$ 83
Derivative, Net Liability Position, Aggregate Fair Value	287
Collateral Posted, Aggregate Fair Value	88
Letters of Credit Outstanding, Amount	64
Restricted Cash and Cash Equivalents, Current	24
Collateral cash requirement	256
Derivative Liability, Fair Value of Collateral	15
Natural Gas [Member]
Derivative Instruments and Hedges, Liabilities	6			7
4911 Electric Services [Member]
Derivative Instruments and Hedges, Liabilities	117			104
Liabilities, Total [Member]
Derivative Instruments and Hedges, Liabilities	$ 123			$ 111