Fair Value Measurements, Recurring (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Assets:
Investments of certain benefit plans	$ 1,729	$ 1,487
Fair Value Measurements (Textual)
Fair value asset transfers Level 1 to Level 2	0	0	0
Fair value asset transfers Level 2 to Level 1	0	0	0
Fair Value, Measurements, Recurring [Member]
Assets:
Commodity derivative contracts	135	176
Physical purchase contracts	11	(2)
Investments of certain benefit plans	98	95
Foreign currency contracts	1
Other investments	0	0
Liabilities:
Commodity derivative contracts	13	25
Biofuels blending obligation	10
Foreign currency contracts	1	3
Fair Value, Inputs, Level 1 [Member]
Assets:
Investments of certain benefit plans	1,035	892
Fair Value, Inputs, Level 1 [Member] \| Fair Value, Measurements, Recurring [Member]
Assets:
Commodity derivative contracts	1,270	2,038
Physical purchase contracts	0	0
Investments of certain benefit plans	87	84
Foreign currency contracts	1
Other investments	0	0
Liabilities:
Commodity derivative contracts	1,138	1,864
Biofuels blending obligation	10
Foreign currency contracts	1	3
Fair Value, Inputs, Level 2 [Member]
Assets:
Investments of certain benefit plans	694	595
Fair Value, Inputs, Level 2 [Member] \| Fair Value, Measurements, Recurring [Member]
Assets:
Commodity derivative contracts	60	78
Physical purchase contracts	11	(2)
Investments of certain benefit plans	0	0
Foreign currency contracts	0
Other investments	0	0
Liabilities:
Commodity derivative contracts	70	101
Biofuels blending obligation	0
Foreign currency contracts	0	0
Fair Value, Inputs, Level 3 [Member]
Assets:
Investments of certain benefit plans	0	0
Fair Value, Inputs, Level 3 [Member] \| Fair Value, Measurements, Recurring [Member]
Assets:
Commodity derivative contracts	0	0
Physical purchase contracts	0	0
Investments of certain benefit plans	11	11
Foreign currency contracts	0
Other investments	0	0
Liabilities:
Commodity derivative contracts	0	0
Biofuels blending obligation	0
Foreign currency contracts	0	0
Netting [Member]
Fair Value Measurements (Textual)
Cash collateral deposits with brokers	127	136
Netting [Member] \| Fair Value, Measurements, Recurring [Member]
Assets:
Commodity derivative contracts	(1,195)	(1,940)
Physical purchase contracts	0	0
Investments of certain benefit plans	0	0
Foreign currency contracts	0
Other investments	0	0
Liabilities:
Commodity derivative contracts	(1,195)	(1,940)
Biofuels blending obligation	0
Foreign currency contracts	$ 0	$ 0

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Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets.

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Fair values as of the balance sheet date of all assets resulting from contracts that meet the criteria of being accounted for as derivative instruments, net of the effects of master netting arrangements.

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Fair value as of the balance sheet date of price risk derivative assets, which includes all such derivative instruments in hedging and nonhedging relationships that are recognized as assets on the balance sheet.

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