Supplemental Oil and Gas Disclosures - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2012 MBbls MMBoe	Dec. 31, 2011 MBbls	Dec. 31, 2010 MBbls
Reserve Quantities [Line Items]
Anticipated Timing of Inclusion of Costs in Amortization Calculation	Five to ten years
Purchases of minerals in-place	73
Extensions, discoveries and other additions	372
Percentage of estimated proved developed reserves classified as proved not producing	15.00%
Proved reserve balance	(298,712)	(59,086)	(11,370)
United States [Member]
Reserve Quantities [Line Items]
Extensions, discoveries and other additions	218
Permian and GOM [Member]
Reserve Quantities [Line Items]
Extensions, discoveries and other additions	23
Other Us Regions [Member]
Reserve Quantities [Line Items]
Extensions, discoveries and other additions	14
Canada [Member]
Reserve Quantities [Line Items]
Extensions, discoveries and other additions	69
Egypt [Member]
Reserve Quantities [Line Items]
Extensions, discoveries and other additions	46
Australia [Member]
Reserve Quantities [Line Items]
Extensions, discoveries and other additions	36
Argentina and North Sea [ Member]
Reserve Quantities [Line Items]
Extensions, discoveries and other additions	5
Cordillera Energy Partners [Member]
Reserve Quantities [Line Items]
Purchases of minerals in-place	68
Other Acquisitions [Member]
Reserve Quantities [Line Items]
Purchases of minerals in-place	6
Permian Basin [Member]
Reserve Quantities [Line Items]
Extensions, discoveries and other additions	104
Anadarko Basin [Member]
Reserve Quantities [Line Items]
Extensions, discoveries and other additions	77