Derivative Instruments - Effect of Derivative Instruments - Statement of Income Table (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Derivative [Line Items]
Unrealized (gains) losses on derivatives, net	$ 246	$ (142)
Total (gains) losses on derivatives, net	191	(284)
Gathering, Processing, and Marketing Sales and Gains (Losses) on Derivatives, Net [Member]
Derivative [Line Items]
Realized (gains) losses on derivatives, net	(47)	(139)
Unrealized (gains) losses on derivatives, net	246	(142)
Total (gains) losses on derivatives, net	199	(281)
Commodity Derivatives [Member] | Gathering, Processing, and Marketing Sales [Member]
Derivative [Line Items]
Realized (gains) losses on derivatives, net	4	(2)
Unrealized (gains) losses on derivatives, net	4	5
Total (gains) losses on derivatives, net	8	3
Commodity Derivatives [Member] | (Gains) Losses on Derivatives, net [Member]
Derivative [Line Items]
Realized (gains) losses on derivatives, net	(51)	(137)
Unrealized (gains) losses on derivatives, net	334	89
Total (gains) losses on derivatives, net	283	(48)
Interest-Rate and Other Derivatives [Member] | (Gains) Losses on Derivatives, net [Member]
Derivative [Line Items]
Unrealized (gains) losses on derivatives, net	(92)	(236)
Total (gains) losses on derivatives, net	$ (92)	$ (236)