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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, 2015
Dec. 31, 2014
Dec. 31, 2013
Standardized Measure of Discounted Future Net Cash Flows      
January 1 $ 3,981,661 $ 3,727,559 $ 2,259,897
Net changes in prices and production costs (3,201,195) (588,212) 254,979
Net changes in future development costs 150,333 (61,760) 57,566
Sales of oil and natural gas, net (477,755) (979,938) (857,540)
Purchases of reserves in place 14,378 38,035 858,382
Sales of reserves in place 0 (251,002) 0
Revisions of previous quantity estimates (946,729) (604,651) 99,954
Previously estimated development costs incurred 216,981 249,926 373,912
Accretion of discount 548,141 548,690 346,068
Net change in income taxes 1,391,358 259,592 (774,910)
Changes in timing and other (172,681) (107,585) (1,951)
December 31 1,914,330 3,981,661 3,727,559
Extensions      
Standardized Measure of Discounted Future Net Cash Flows      
Proved oil and gas reserves increase 409,838 1,751,007 1,111,202
Discoveries      
Standardized Measure of Discounted Future Net Cash Flows      
Proved oil and gas reserves increase $ 0 $ 0 $ 0