Derivative Financial Instruments Recorded in Balance Sheet (Details) $ in Millions	Dec. 31, 2016 USD ($) MMBTU $ / bbl $ / MMBTU	Dec. 31, 2015 USD ($)
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Asset	$ 161	$ 638
Derivative Asset Not Designated As Hedging Instrument, Fair Value	236	1,027
Derivative Asset, Fair Value, Gross Liability	(161)	(638)
Derivative Asset, Current	75	284
Derivative Asset, Noncurrent	0	105
Derivative Liability, Current	(97)	(13)
Derivative Liability, Noncurrent	(3)	(9)
Derivative Liability, Fair Value, Gross Liability	(261)	(660)
Derivative Liability Not Designated as Hedging Instrument Fair Value	(261)	(660)
Natural gas contracts [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Asset	10	0
Derivative Liability, Fair Value, Total	(54)
Derivative Asset Not Designated As Hedging Instrument, Fair Value	10	22
Derivative Asset, Fair Value, Gross Liability	(10)	0
Derivative Asset, Current	0	22
Derivative Asset, Noncurrent	0	0
Derivative Liability, Current	(51)	0
Derivative Liability, Noncurrent	(3)	0
Derivative Liability, Fair Value, Gross Liability	(64)	0
Derivative Liability Not Designated as Hedging Instrument Fair Value	(64)	0
Oil contracts [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Asset	151	638
Derivative Asset, Fair Value, Total	(29)
Derivative Asset Not Designated As Hedging Instrument, Fair Value	226	1,005
Derivative Asset, Fair Value, Gross Liability	(151)	(638)
Derivative Asset, Current	75	262
Derivative Asset, Noncurrent	0	105
Derivative Liability, Current	(46)	(13)
Derivative Liability, Noncurrent	0	(9)
Derivative Liability, Fair Value, Gross Liability	(197)	(660)
Derivative Liability Not Designated as Hedging Instrument Fair Value	(197)	$ (660)
Year2017 [Member] | Collars [Member] | Natural gas contracts [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Total	$ (27)
Volume In Mmbtus | MMBTU	53,860,000
Derivative, Average Floor Price | $ / MMBTU	2.82
Derivative, Average Cap Price | $ / MMBTU	3.23
Year2017 [Member] | Swap [Member] | Natural gas contracts [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Total	$ (24)
Volume In Mmbtus | MMBTU	27,375,000
Derivative, Swap Type, Average Fixed Price | $ / MMBTU	2.73
Year2017 [Member] | Swap [Member] | Oil contracts [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Total	$ (67)
Derivative, Swap Type, Average Fixed Price | $ / bbl	45.430
Year2018 [Member] | Collars [Member] | Natural gas contracts [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Total	$ (2)
Volume In Mmbtus | MMBTU	18,150,000
Derivative, Average Floor Price | $ / MMBTU	3
Derivative, Average Cap Price | $ / MMBTU	3.55
Year2018 [Member] | Swap [Member] | Natural gas contracts [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Total	$ (1)
Volume In Mmbtus | MMBTU	10,950,000
Derivative, Swap Type, Average Fixed Price | $ / MMBTU	3.01