Supplemental Oil and Gas Disclosures - Summary of Oil and Gas Property Costs Not Being Amortized (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Property acquisition costs	$ 6,998
Exploration and development	1,550
Capitalized interest	206
Costs incurred	8,754
2012 [Member]
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Property acquisition costs	3,558
Exploration and development	1,108
Capitalized interest	48
Costs incurred	4,714
2011 [Member]
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Property acquisition costs	981
Exploration and development	368
Capitalized interest	27
Costs incurred	1,376
2010 [Member]
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Property acquisition costs	2,146
Exploration and development	16
Capitalized interest	55
Costs incurred	2,217
2009 and Prior [Member]
Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items]
Property acquisition costs	313
Exploration and development	58
Capitalized interest	76
Costs incurred	$ 447