Derivatives and Risk Management Activities - Offsetting (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivative Asset Positions
Gross Position - Asset	$ 90	$ 108
Netting Adjustment	(239)	(350)
Cash collateral paid	212	410
Net Position - Asset	63	168
Derivative Liability Positions
Gross Position - Liability	(425)	(481)
Netting Adjustment	239	350
Net Position - Liability	(186)	(131)
Other current assets
Derivative Asset Positions
Net Position - Asset	62	167
Other long-term assets, net
Derivative Asset Positions
Net Position - Asset	1	1
Other current liabilities
Derivative Liability Positions
Net Position - Liability	(151)	(40)
Other long-term liabilities and deferred credits
Derivative Liability Positions
Net Position - Liability	$ (35)	$ (91)