Derivative Instruments - Effect of Derivative Instruments - Statement of Income Table (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Derivative [Line Items]
Total (gains) losses on derivatives, net	$ 207		$ (392)		$ (308)
Commodity Derivatives [Member] | Gathering, Processing, and Marketing Sales [Member]
Derivative [Line Items]
Total (gains) losses on derivatives, net	10	[1]	6	[1]	18	[1]
Commodity Derivatives [Member] | (Gains) Losses on Derivatives, net [Member]
Derivative [Line Items]
Total (gains) losses on derivatives, net	(589)		141		(387)
Interest-Rate and Other Derivatives [Member] | (Gains) Losses on Derivatives, net [Member]
Derivative [Line Items]
Total (gains) losses on derivatives, net	$ 786		$ (539)		$ 61

[1] Represents the effect of Marketing and Trading Derivative Activities.