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Supplemental Information on Natural Gas and Oil Exploration, Development and Production Activities (Unaudited) - Changes in Discounted Future Net Cash Flows (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Revisions of previous estimates      
Changes in prices and production costs $ (4,868,371) $ (6,560,107) $ 2,959,488
Revisions in quantities (345,073) (12,741) 667,763
Changes in future development and abandonment costs 107,899 104,585 (686,632)
Net change in income taxes 797,816 1,125,639 (1,075,867)
Accretion of discount 756,083 1,317,349 814,725
Additions to proved reserves from extensions, discoveries and improved recovery 280,441 552,710 2,543,296
Natural gas, NGLs and oil sales, net of production costs (402,450) (881,883) (1,547,580)
Actual development costs incurred during the period 384,530 676,520 851,188
Sales of reserves in place (394,125) (688,937) (226,953)
Timing and other (99,001) (120,156) (349,048)
Net change for the year (3,782,251) (4,487,021) 3,950,380
Beginning of year 6,628,573 11,115,594 7,165,214
End of year $ 2,846,322 $ 6,628,573 $ 11,115,594