Financial and Other Derivative Instruments (Netting Offsets Reconciliation to Balance Sheet) (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Derivative Assets:
Derivative Assets	$ 549,000,000	$ 711,000,000
Collateral adjustment	(7,000,000)	(2,000,000)
Derivative asset, current	47,000,000	129,000,000
Derivative assets, noncurrent	34,000,000	54,000,000
Derivative Liabilities:
Derivative Liabilities	(656,000,000)	(705,000,000)
Counterparty netting	28,000,000	36,000,000
Derivative liabilities, current	(69,000,000)	(57,000,000)
Derivative liabilities, noncurrent	(98,000,000)	(86,000,000)
Current derivative asset
Derivative Assets:
Derivative Assets	447,000,000	570,000,000
Counterparty netting	(396,000,000)	(441,000,000)
Collateral adjustment	(4,000,000)	0
Derivative asset, current	47,000,000	129,000,000
Noncurrent derivative asset
Derivative Assets:
Derivative Assets	102,000,000	141,000,000
Counterparty netting	(65,000,000)	(85,000,000)
Collateral adjustment	(3,000,000)	(2,000,000)
Derivative assets, noncurrent	34,000,000	54,000,000
Current derivative liability
Derivative Liabilities:
Derivative Liabilities	(493,000,000)	(521,000,000)
Collateral adjustment	396,000,000	441,000,000
Counterparty netting	28,000,000	23,000,000
Derivative liabilities, current	(69,000,000)	(57,000,000)
Noncurrent derivative liability
Derivative Liabilities:
Derivative Liabilities	(163,000,000)	(184,000,000)
Collateral adjustment	65,000,000	85,000,000
Counterparty netting	0	13,000,000
Derivative liabilities, noncurrent	$ (98,000,000)	$ (86,000,000)