Derivative Financial Instruments Recorded in Balance Sheet (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Asset	$ 599	$ 11
Derivative Asset Not Designated As Hedging Instrument, Fair Value	1,220	37
Derivative Asset, Fair Value, Gross Liability	(599)	(11)
Derivative Asset, Current	431us-gaap_DerivativeAssetsCurrent	0us-gaap_DerivativeAssetsCurrent
Derivative Asset, Noncurrent	190us-gaap_DerivativeAssetsNoncurrent	26us-gaap_DerivativeAssetsNoncurrent
Derivative Liability, Current	(8)us-gaap_DerivativeLiabilitiesCurrent	(62)us-gaap_DerivativeLiabilitiesCurrent
Derivative Liability, Noncurrent	0us-gaap_DerivativeLiabilitiesNoncurrent	0us-gaap_DerivativeLiabilitiesNoncurrent
Derivative Liability, Fair Value, Gross Liability	(607)	(73)
Derivative Liability Not Designated as Hedging Instrument Fair Value	(607)	(73)
Natural gas contracts [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Asset	2	2
Derivative Asset, Fair Value, Total	103
Derivative Asset Not Designated As Hedging Instrument, Fair Value	105	11
Derivative Asset, Fair Value, Gross Liability	(2)	(2)
Derivative Asset, Current	99	0
Derivative Asset, Noncurrent	4	9
Derivative Liability, Current	0	(20)
Derivative Liability, Noncurrent	0	0
Derivative Liability, Fair Value, Gross Liability	(2)	(22)
Derivative Liability Not Designated as Hedging Instrument Fair Value	(2)	(22)
Oil contracts [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Asset	597	9
Derivative Asset, Fair Value, Total	510
Derivative Asset Not Designated As Hedging Instrument, Fair Value	1,115	26
Derivative Asset, Fair Value, Gross Liability	(597)	(9)
Derivative Asset, Current	332	0
Derivative Asset, Noncurrent	186	17
Derivative Liability, Current	(8)	(42)
Derivative Liability, Noncurrent	0	0
Derivative Liability, Fair Value, Gross Liability	(605)	(51)
Derivative Liability Not Designated as Hedging Instrument Fair Value	$ (605)	$ (51)

Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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