Risk Management and Trading Activities (Details 2) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Assets
Collateral and counterparty netting	$ (306)	$ (507)
Total derivative contracts (Assets)	1,527	1,463
Other assets measured at fair value on a recurring basis	39	72
Total assets	1,566	1,535
Liabilities
Collateral and counterparty netting	302	328
Total derivative contracts (Liabilities)	(2,008)	(2,707)
Other liabilities measured at fair value on a recurring basis	(52)	0
Total liabilities	(2,060)	(2,707)
Level 1 [Member]
Assets
Collateral and counterparty netting	(33)	(1)
Total derivative contracts (Assets)	102	64
Other assets measured at fair value on a recurring basis	7	20
Total assets	109	84
Liabilities
Collateral and counterparty netting	33	1
Total derivative contracts (Liabilities)	(158)	(323)
Other liabilities measured at fair value on a recurring basis	0	0
Total liabilities	(158)	(323)
Level 2 [Member]
Assets
Collateral and counterparty netting	(148)	(274)
Total derivative contracts (Assets)	1,106	1,052
Other assets measured at fair value on a recurring basis	34	49
Total assets	1,140	1,101
Liabilities
Collateral and counterparty netting	148	274
Total derivative contracts (Liabilities)	(1,386)	(2,267)
Other liabilities measured at fair value on a recurring basis	(52)	0
Total liabilities	(1,438)	(2,267)
Level 3 [Member]
Assets
Collateral and counterparty netting	(4)	(19)
Total derivative contracts (Assets)	507	864
Other assets measured at fair value on a recurring basis	0	3
Total assets	507	867
Liabilities
Collateral and counterparty netting	4	19
Total derivative contracts (Liabilities)	(648)	(455)
Other liabilities measured at fair value on a recurring basis	(2)	0
Total liabilities	(650)	(455)
Collateral and counterparty netting elimination [Member]
Assets
Collateral and counterparty netting	(121)	(213)
Total derivative contracts (Assets)	(188)	(517)
Other assets measured at fair value on a recurring basis	(2)	0
Total assets	(190)	(517)
Liabilities
Collateral and counterparty netting	117	34
Total derivative contracts (Liabilities)	184	338
Other liabilities measured at fair value on a recurring basis	2	0
Total liabilities	186	338
Commodity Contract [Member]
Assets
Total derivative contracts (Assets)	1,767	1,952
Liabilities
Total derivative contracts (Liabilities)	(2,275)	(3,013)
Commodity Contract [Member] | Level 1 [Member]
Assets
Total derivative contracts (Assets)	135	65
Liabilities
Total derivative contracts (Liabilities)	(191)	(324)
Commodity Contract [Member] | Level 2 [Member]
Assets
Total derivative contracts (Assets)	1,188	1,308
Liabilities
Total derivative contracts (Liabilities)	(1,501)	(2,519)
Commodity Contract [Member] | Level 3 [Member]
Assets
Total derivative contracts (Assets)	511	883
Liabilities
Total derivative contracts (Liabilities)	(650)	(474)
Commodity Contract [Member] | Collateral and counterparty netting elimination [Member]
Assets
Total derivative contracts (Assets)	(67)	(304)
Liabilities
Total derivative contracts (Liabilities)	67	304
Foreign Exchange Contract [Member]
Assets
Total derivative contracts (Assets)		1
Liabilities
Total derivative contracts (Liabilities)	(15)	(12)
Foreign Exchange Contract [Member] | Level 1 [Member]
Assets
Total derivative contracts (Assets)		0
Liabilities
Total derivative contracts (Liabilities)	0	0
Foreign Exchange Contract [Member] | Level 2 [Member]
Assets
Total derivative contracts (Assets)		1
Liabilities
Total derivative contracts (Liabilities)	(15)	(12)
Foreign Exchange Contract [Member] | Level 3 [Member]
Assets
Total derivative contracts (Assets)		0
Liabilities
Total derivative contracts (Liabilities)	0	0
Foreign Exchange Contract [Member] | Collateral and counterparty netting elimination [Member]
Assets
Total derivative contracts (Assets)		0
Liabilities
Total derivative contracts (Liabilities)	0	0
Interest Rate and Other [Member]
Assets
Total derivative contracts (Assets)	66	17
Interest Rate and Other [Member] | Level 1 [Member]
Assets
Total derivative contracts (Assets)	0	0
Interest Rate and Other [Member] | Level 2 [Member]
Assets
Total derivative contracts (Assets)	66	17
Interest Rate and Other [Member] | Level 3 [Member]
Assets
Total derivative contracts (Assets)	0	0
Interest Rate and Other [Member] | Collateral and counterparty netting elimination [Member]
Assets
Total derivative contracts (Assets)	0	0
Other [Member]
Liabilities
Total derivative contracts (Liabilities)	(20)	(10)
Other [Member] | Level 1 [Member]
Liabilities
Total derivative contracts (Liabilities)	0	0
Other [Member] | Level 2 [Member]
Liabilities
Total derivative contracts (Liabilities)	(18)	(10)
Other [Member] | Level 3 [Member]
Liabilities
Total derivative contracts (Liabilities)	(2)	0
Other [Member] | Collateral and counterparty netting elimination [Member]
Liabilities
Total derivative contracts (Liabilities)	$ 0	$ 0