Price Risk Management Price Risk Management (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	$ 15			$ 15
Collateral, Master Netting Arrangements, Letters of Credit	14			14		$ 14
Derivative, Collateral, Right to Reclaim Cash	1			1
Net gain or (loss) recognized in the statement of income offset by regulatory accounting	20	$ 34		36	$ 150
Derivative, Net Liability Position, Aggregate Fair Value	227			227
Collateral Posted, Aggregate Fair Value	36			36
Letters of Credit Outstanding, Amount	31			31
Restricted Cash and Cash Equivalents, Current	5			5
Collateral cash requirement	219			219
Natural Gas [Member]
Derivative Instruments and Hedges, Liabilities	4			4		7
4911 Electric Services [Member]
Derivative Instruments and Hedges, Liabilities	139			139		104
Liabilities, Total [Member]
Derivative Instruments and Hedges, Liabilities	$ 143			$ 143		$ 111