Acquisitions and Divestitures - Assets Acquired and Liabilities Assumed as of Acquisition Date (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Business Acquisition [Line Items]
Proved properties	$ 78,383	$ 67,805
Unproved properties	8,754	5,530
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
Mobil North Sea Limited [Member]
Business Acquisition [Line Items]
Current assets	219
Proved properties	2,341
Unproved properties	476
Gathering, transmission, and processing facilities	338
Goodwill	84
Total assets acquired	3,458
Current liabilities	148
Asset retirement obligation	517
Deferred income tax liabilities	1,546
Other long-term obligations	1
Total liabilities assumed	2,212
Net assets acquired	$ 1,246