Acquisitions and Divestitures - Assets Acquired and Liabilities Assumed as of Acquisition Date (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Business Acquisition [Line Items]
Proved properties	$ 80,631	$ 78,383
Unproved properties	8,574	8,754
Cordillera Energy Partners [Member]
Business Acquisition [Line Items]
Current assets	39
Proved properties	1,040
Unproved properties	2,299
Gathering, transmission, and processing facilities	1
Goodwill	173
Deferred tax asset	64
Total assets acquired	3,616
Current liabilities	88
Deferred income tax liabilities	237
Other long-term obligations	5
Total liabilities assumed	330
Net assets acquired	$ 3,286