Financial and Other Derivative Instruments (Netting Offsets Reconciliation to Balance Sheet) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivative Assets:
Derivative Assets	$ 652	$ 549
Collateral adjustment	(9)	(7)
Derivative asset, current	103	47
Derivative assets, noncurrent	51	34
Derivative Liabilities:
Derivative Liabilities	(657)	(656)
Counterparty netting	22	28
Derivative liabilities, current	(99)	(69)
Derivative liabilities, noncurrent	(47)	(98)
Current derivative asset
Derivative Assets:
Derivative Assets	540	447
Counterparty netting	(437)	(396)
Collateral adjustment	0	(4)
Derivative asset, current	103	47
Noncurrent derivative asset
Derivative Assets:
Derivative Assets	112	102
Counterparty netting	(52)	(65)
Collateral adjustment	(9)	(3)
Derivative assets, noncurrent	51	34
Current derivative liability
Derivative Liabilities:
Derivative Liabilities	(558)	(493)
Collateral adjustment	437	396
Counterparty netting	22	28
Derivative liabilities, current	(99)	(69)
Noncurrent derivative liability
Derivative Liabilities:
Derivative Liabilities	(99)	(163)
Collateral adjustment	52	65
Counterparty netting	0	0
Derivative liabilities, noncurrent	$ (47)	$ (98)