Financial and Other Derivative Instruments (Netting Offsets Reconciliation to Balance Sheet) (Details) - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Derivative Assets
Derivative Assets	$ 388,000,000	$ 652,000,000
Collateral adjustment	0	(9,000,000)
Derivative assets, current	78,000,000	103,000,000
Derivative assets, noncurrent	45,000,000	51,000,000
Derivative Liabilities
Derivative Liabilities	(372,000,000)	(657,000,000)
Collateral adjustment	0	22,000,000
Derivative liabilities, current	(43,000,000)	(99,000,000)
Derivative liabilities, noncurrent	(64,000,000)	(47,000,000)
Current derivative assets
Derivative Assets
Derivative Assets	291,000,000	540,000,000
Counterparty netting	(213,000,000)	(437,000,000)
Collateral adjustment	0	0
Derivative assets, current	78,000,000	103,000,000
Noncurrent derivative assets
Derivative Assets
Derivative Assets	97,000,000	112,000,000
Counterparty netting	(52,000,000)	(52,000,000)
Collateral adjustment	0	(9,000,000)
Derivative assets, noncurrent	45,000,000	51,000,000
Current derivative liabilities
Derivative Liabilities
Derivative Liabilities	(256,000,000)	(558,000,000)
Counterparty netting	213,000,000	437,000,000
Collateral adjustment	0	22,000,000
Derivative liabilities, current	(43,000,000)	(99,000,000)
Noncurrent derivative liabilities
Derivative Liabilities
Derivative Liabilities	(116,000,000)	(99,000,000)
Counterparty netting	52,000,000	52,000,000
Collateral adjustment	0	0
Derivative liabilities, noncurrent	$ (64,000,000)	$ (47,000,000)