Annual report pursuant to Section 13 and 15(d)

Schedule of Changes In Standardized Measure (Detail)

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Schedule of Changes In Standardized Measure (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Reserve Quantities [Line Items]      
Changes in Standardized Measure Standardized measure, beginning of year $ 2,006,377 $ 1,179,072 $ 660,396
Sales and transfers of oil and gas produced, net of production costs (620,437) (729,574) (521,551)
Net changes in price, net of future production costs (224,260) 634,174 367,575
Extensions and discoveries, net of future production and development costs 181,870 219,924 143,612
Changes in estimated future development costs (103,320) (4,572) (59,124)
Previously estimated development costs incurred 332,939 173,911 97,188
Revisions of quantity estimates (128,075) 204,988 94,735
Accretion of discount 231,144 135,791 68,862
Net change in income taxes 99,684 (398,204) (221,226)
Purchases of reserves in-place 270,168 483,286 624,302
Sales of reserves in-place (16,105)    
Changes in production rates due to timing and other (183,608) 107,581 (75,697)
Net increase (decrease) in standardized measure (160,000) 827,305 518,676
Standardized measure, end of year $ 1,846,377 $ 2,006,377 $ 1,179,072