Risk Management and Trading Activities - Fair Values for Corporation's Net Physical Derivative Financial Assets and (Liabilities) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Assets
Total derivative contracts (Assets)	$ 150	$ 243
Net derivative contracts (Assets)	604	569
Other assets measured at fair value on a recurring basis	8	52
Total assets measured at fair value on a recurring basis	612	621
Liabilities
Total derivative contracts (Liabilities)	(130)	(101)
Net derivative contracts (Liabilities)	(612)	(567)
Other liabilities measured at fair value on a recurring basis	(22)	(42)
Total liabilities measured at fair value on a recurring basis	(634)	(609)
Other fair value measurement disclosures Long-term debt	(6,606)	(8,887)
Commodity [Member]
Assets
Total derivative contracts (Assets)	637	558
Liabilities
Total derivative contracts (Liabilities)	(691)	(646)
Interest rate and other [Member]
Assets
Total derivative contracts (Assets)	47	92
Net derivative contracts (Assets)	47	92
Collateral and counterparty netting [Member]
Assets
Net derivative contracts (Assets)	(80)	(81)
Liabilities
Net derivative contracts (Liabilities)	80	81
Other [Member]
Liabilities
Total derivative contracts (Liabilities)	(1)	(2)
Level 1 [Member]
Assets
Net derivative contracts (Assets)	109	77
Other assets measured at fair value on a recurring basis	8	5
Total assets measured at fair value on a recurring basis	117	82
Liabilities
Net derivative contracts (Liabilities)	(43)	(61)
Other liabilities measured at fair value on a recurring basis	(22)	(40)
Total liabilities measured at fair value on a recurring basis	(65)	(101)
Level 1 [Member] | Commodity [Member]
Assets
Total derivative contracts (Assets)	131	94
Liabilities
Total derivative contracts (Liabilities)	(75)	(83)
Level 1 [Member] | Interest rate and other [Member]
Assets
Total derivative contracts (Assets)	10	6
Level 1 [Member] | Collateral and counterparty netting [Member]
Assets
Net derivative contracts (Assets)	(32)	(23)
Liabilities
Net derivative contracts (Liabilities)	32	23
Level 1 [Member] | Other [Member]
Liabilities
Total derivative contracts (Liabilities)		(1)
Level 2 [Member]
Assets
Net derivative contracts (Assets)	410	477
Other assets measured at fair value on a recurring basis		49
Total assets measured at fair value on a recurring basis	410	526
Liabilities
Net derivative contracts (Liabilities)	(582)	(605)
Other liabilities measured at fair value on a recurring basis		(2)
Total liabilities measured at fair value on a recurring basis	(582)	(607)
Other fair value measurement disclosures Long-term debt	(6,606)	(8,887)
Level 2 [Member] | Commodity [Member]
Assets
Total derivative contracts (Assets)	417	445
Liabilities
Total derivative contracts (Liabilities)	(627)	(657)
Level 2 [Member] | Interest rate and other [Member]
Assets
Total derivative contracts (Assets)	40	86
Level 2 [Member] | Collateral and counterparty netting [Member]
Assets
Net derivative contracts (Assets)	(47)	(54)
Liabilities
Net derivative contracts (Liabilities)	47	54
Level 2 [Member] | Other [Member]
Liabilities
Total derivative contracts (Liabilities)	(2)	(2)
Level 3 [Member]
Assets
Net derivative contracts (Assets)	151	240
Total assets measured at fair value on a recurring basis	151	240
Liabilities
Net derivative contracts (Liabilities)	(129)	(97)
Other liabilities measured at fair value on a recurring basis		(2)
Total liabilities measured at fair value on a recurring basis	(129)	(99)
Level 3 [Member] | Commodity [Member]
Assets
Total derivative contracts (Assets)	150	243
Liabilities
Total derivative contracts (Liabilities)	(130)	(101)
Level 3 [Member] | Interest rate and other [Member]
Assets
Total derivative contracts (Assets)	2	1
Level 3 [Member] | Collateral and counterparty netting [Member]
Assets
Net derivative contracts (Assets)	(1)	(4)
Liabilities
Net derivative contracts (Liabilities)	1	4
Counterparty netting [Member]
Assets
Net derivative contracts (Assets)	(43)	(191)
Other assets measured at fair value on a recurring basis		(2)
Total assets measured at fair value on a recurring basis	(43)	(193)
Liabilities
Net derivative contracts (Liabilities)	43	191
Other liabilities measured at fair value on a recurring basis		2
Total liabilities measured at fair value on a recurring basis	43	193
Counterparty netting [Member] | Commodity [Member]
Assets
Total derivative contracts (Assets)	(42)	(190)
Liabilities
Total derivative contracts (Liabilities)	42	190
Counterparty netting [Member] | Interest rate and other [Member]
Assets
Total derivative contracts (Assets)	(1)	(1)
Counterparty netting [Member] | Other [Member]
Liabilities
Total derivative contracts (Liabilities)	1	1
Collateral [Member]
Assets
Net derivative contracts (Assets)	(23)	(34)
Total assets measured at fair value on a recurring basis	(23)	(34)
Liabilities
Net derivative contracts (Liabilities)	99	5
Total liabilities measured at fair value on a recurring basis	99	5
Collateral [Member] | Commodity [Member]
Assets
Total derivative contracts (Assets)	(19)	(34)
Liabilities
Total derivative contracts (Liabilities)	99	5
Collateral [Member] | Interest rate and other [Member]
Assets
Total derivative contracts (Assets)	$ (4)