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Supplemental Oil and Gas Reserve Information - Unaudited - Standardized Measure of Discounted Future Cash Flows (Detail) - USD ($)
$ in Thousands
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Oil and Gas Exploration and Production Industries Disclosures [Abstract]        
Future cash inflows $ 8,582,831 $ 21,656,832 $ 19,063,500  
Future production costs (3,842,517) (7,094,426) (5,473,767)  
Future development costs (909,562) (2,563,062) (1,904,095)  
Future income tax expense (225,662) (3,188,389) (3,628,977)  
Future net cash flows 3,605,090 8,810,955 8,056,661  
10% annual discount for estimated timing of cash flows (1,690,760) (4,829,294) (4,329,102)  
Standardized measure of discounted future net cash flows $ 1,914,330 $ 3,981,661 $ 3,727,559 $ 2,259,897