Price Risk Management Price Risk Management (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Sep. 30, 2014	Jun. 30, 2016	Jun. 30, 2015	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	18	$ 33		16	$ 116
Derivative, Net Liability Position, Aggregate Fair Value	249			249
Collateral Posted, Aggregate Fair Value	60			60
Letters of Credit Outstanding, Amount	49			49
Restricted Cash and Cash Equivalents, Current	11			11
Collateral cash requirement	230			230
Natural Gas [Member]
Derivative Instruments and Hedges, Liabilities	4			4		$ 7
4911 Electric Services [Member]
Derivative Instruments and Hedges, Liabilities	144			144		104
Liabilities, Total [Member]
Derivative Instruments and Hedges, Liabilities	$ 148			$ 148		$ 111