Price Risk Management (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	$ 136	$ 115
Derivative, Collateral, Right to Reclaim Cash	11	11
Net gain or loss recognized in the statement of income offset by regulatory accounting	82	13	$ 160
Derivative, Net Liability Position, Aggregate Fair Value	205
Collateral Already Posted, Aggregate Fair Value	31
Collateral Aggregate Fair Value	202
Margin Deposit Assets	0
Electricity [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	130	112
Natural Gas [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	$ 6	$ 3