Price Risk Management (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Oct. 16, 2015
Derivative [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	$ 111	$ 72
Derivative, Collateral, Right to Reclaim Cash	14	11		$ 19
Derivative, Collateral, Master Netting Arrangements, Letters of Credit	61
Net gain or loss recognized in the statement of income offset by regulatory accounting	160	83	$ 120
Derivative, Net Liability Position, Aggregate Fair Value	278
Collateral Already Posted, Aggregate Fair Value	80
Collateral Aggregate Fair Value	255
Margin Deposit Assets	14
Electricity [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	104	55
Natural Gas [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	$ 7	$ 17