Risk Management and Trading Activities (Details 2) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Assets
Collateral and counterparty netting	$ (431)	$ (507)
Total derivative contracts (Assets)	1,229	1,463
Other assets measured at fair value on a recurring basis	99	72
Total assets	1,328	1,535
Liabilities
Collateral and counterparty netting	383	328
Total derivative contracts (Liabilities)	(2,107)	(2,707)
Other liabilities measured at fair value on a recurring basis	(58)	0
Total liabilities	(2,165)	(2,707)
Commodity Contracts [Member]
Assets
Total derivative contracts (Assets)	1,586	1,952
Liabilities
Total derivative contracts (Liabilities)	(2,387)	(3,013)
Commodity Contracts [Member] | Level 1 [Member]
Assets
Total derivative contracts (Assets)	283	65
Liabilities
Total derivative contracts (Liabilities)	(490)	(324)
Commodity Contracts [Member] | Level 2 [Member]
Assets
Total derivative contracts (Assets)	1,288	1,308
Liabilities
Total derivative contracts (Liabilities)	(1,645)	(2,519)
Commodity Contracts [Member] | Level 3 [Member]
Assets
Total derivative contracts (Assets)	320	883
Liabilities
Total derivative contracts (Liabilities)	(557)	(474)
Commodity Contracts [Member] | Collateral and counterparty netting elimination [Member]
Assets
Total derivative contracts (Assets)	(305)	(304)
Liabilities
Total derivative contracts (Liabilities)	305	304
Foreign Exchange Contracts [Member]
Assets
Total derivative contracts (Assets)	1	1
Liabilities
Total derivative contracts (Liabilities)	(36)	(12)
Foreign Exchange Contracts [Member] | Level 1 [Member]
Assets
Total derivative contracts (Assets)	0	0
Liabilities
Total derivative contracts (Liabilities)	0	0
Foreign Exchange Contracts [Member] | Level 2 [Member]
Assets
Total derivative contracts (Assets)	1	1
Liabilities
Total derivative contracts (Liabilities)	(36)	(12)
Foreign Exchange Contracts [Member] | Level 3 [Member]
Assets
Total derivative contracts (Assets)	0	0
Liabilities
Total derivative contracts (Liabilities)	0	0
Foreign Exchange Contracts [Member] | Collateral and counterparty netting elimination [Member]
Assets
Total derivative contracts (Assets)	0	0
Liabilities
Total derivative contracts (Liabilities)	0	0
Interest Rate and Other [Member]
Assets
Total derivative contracts (Assets)	73	17
Interest Rate and Other [Member] | Level 1 [Member]
Assets
Total derivative contracts (Assets)	2	0
Interest Rate and Other [Member] | Level 2 [Member]
Assets
Total derivative contracts (Assets)	71	17
Interest Rate and Other [Member] | Level 3 [Member]
Assets
Total derivative contracts (Assets)	1	0
Interest Rate and Other [Member] | Collateral and counterparty netting elimination [Member]
Assets
Total derivative contracts (Assets)	(1)	0
Other [Member]
Liabilities
Total derivative contracts (Liabilities)	(67)	(10)
Other [Member] | Level 1 [Member]
Liabilities
Total derivative contracts (Liabilities)	0	0
Other [Member] | Level 2 [Member]
Liabilities
Total derivative contracts (Liabilities)	(61)	(10)
Other [Member] | Level 3 [Member]
Liabilities
Total derivative contracts (Liabilities)	(7)	0
Other [Member] | Collateral and counterparty netting elimination [Member]
Liabilities
Total derivative contracts (Liabilities)	1	0
Level 1 [Member]
Assets
Collateral and counterparty netting	(58)	(1)
Total derivative contracts (Assets)	227	64
Other assets measured at fair value on a recurring basis	10	20
Total assets	237	84
Liabilities
Collateral and counterparty netting	58	1
Total derivative contracts (Liabilities)	(432)	(323)
Other liabilities measured at fair value on a recurring basis	0	0
Total liabilities	(432)	(323)
Level 2 [Member]
Assets
Collateral and counterparty netting	(220)	(274)
Total derivative contracts (Assets)	1,140	1,052
Other assets measured at fair value on a recurring basis	90	49
Total assets	1,230	1,101
Liabilities
Collateral and counterparty netting	220	274
Total derivative contracts (Liabilities)	(1,522)	(2,267)
Other liabilities measured at fair value on a recurring basis	(58)	0
Total liabilities	(1,580)	(2,267)
Level 3 [Member]
Assets
Collateral and counterparty netting	(20)	(19)
Total derivative contracts (Assets)	301	864
Other assets measured at fair value on a recurring basis	0	3
Total assets	301	867
Liabilities
Collateral and counterparty netting	20	19
Total derivative contracts (Liabilities)	(544)	(455)
Other liabilities measured at fair value on a recurring basis	(1)	0
Total liabilities	(545)	(455)
Collateral and counterparty netting elimination [Member]
Assets
Collateral and counterparty netting	(133)	(213)
Total derivative contracts (Assets)	(439)	(517)
Other assets measured at fair value on a recurring basis	(1)	0
Total assets	(440)	(517)
Liabilities
Collateral and counterparty netting	85	34
Total derivative contracts (Liabilities)	391	338
Other liabilities measured at fair value on a recurring basis	1	0
Total liabilities	$ 392	$ 338