Price Risk Management (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	$ 115	$ 111
Derivative, Collateral, Right to Reclaim Cash	11	14
Net gain or loss recognized in the statement of income offset by regulatory accounting	13	160	$ 83
Derivative, Net Liability Position, Aggregate Fair Value	164
Collateral Already Posted, Aggregate Fair Value	14
Collateral Aggregate Fair Value	149
Margin Deposit Assets	0
Electricity [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	112	104
Natural Gas [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	$ 3	$ 7