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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
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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

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