Acquisitions and Divestitures - Assets Acquired and Liabilities Assumed as of Acquisition Date (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Mar. 31, 2013 Cordillera Energy Partners [Member]
Business Acquisition [Line Items]
Current assets			$ 39
Proved properties	83,287	78,383	1,040
Unproved properties	8,628	8,754	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