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Supplemental Oil and Gas Reserve Information - Unaudited - Standardized Measure of Discounted Future Cash Flows (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Oil and Gas Exploration and Production Industries Disclosures [Abstract]        
Future cash inflows $ 11,636,126 $ 9,426,963 $ 8,582,831  
Future production costs (4,458,418) (3,996,657) (3,842,517)  
Future development costs (992,271) (784,727) (909,562)  
Future income tax expense (580,481) (279,345) (225,662)  
Future net cash flows 5,604,956 4,366,234 3,605,090  
10% annual discount for estimated timing of cash flows (2,304,261) (1,883,169) (1,690,760)  
Standardized measure of discounted future net cash flows $ 3,300,695 $ 2,483,065 $ 1,914,330 $ 3,981,661