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Supplemental Oil and Gas Reserve Information - Unaudited - Changes in Standardized Measure of Discounted Future Net Cash Flows Applicable to Proved Oil and Natural Gas Reserves (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Standardized Measure of Discounted Future Net Cash Flows      
January 1 $ 2,483,065 $ 1,914,330 $ 3,981,661
Net changes in prices and production costs 881,993 (367,527) (3,201,195)
Net changes in future development costs (60,929) 69,992 150,333
Sales of oil and natural gas, net (769,625) (403,739) (477,755)
Purchases of reserves in place 6,518 533,505 14,378
Sales of reserves in place (9,024) (57,770) 0
Revisions of previous quantity estimates (78,942) 333,398 (946,729)
Previously estimated development costs incurred 157,386 91,518 216,981
Accretion of discount 262,776 202,272 548,141
Net change in income taxes (238,354) (36,303) 1,391,358
Changes in timing and other 4,364 37,463 (172,681)
December 31 3,300,695 2,483,065 1,914,330
Extensions      
Standardized Measure of Discounted Future Net Cash Flows      
Proved oil and gas reserves increase 661,467 165,926 409,838
Discoveries      
Standardized Measure of Discounted Future Net Cash Flows      
Proved oil and gas reserves increase $ 0 $ 0 $ 0