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Supplemental Information on Natural Gas and Oil Exploration, Development and Production Activities (Unaudited) - Standardized Measure of Discounted Future Net Cash Flows (Detail) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Extractive Industries [Abstract]        
Future cash inflows $ 35,820,758 $ 48,718,733    
Future costs:        
Production (23,281,268) (23,320,477)    
Development [1] (2,744,539) (3,219,349)    
Future net cash flows before income taxes 9,794,951 22,178,907    
Future income tax expense (1,240,011) (4,179,297)    
Total future net cash flows before 10% discount 8,554,940 17,999,610    
10% annual discount (5,708,618) (11,371,037)    
Standardized measure of discounted future net cash flows $ 2,846,322 $ 6,628,573 $ 11,115,594 $ 7,165,214
[1] 2020 includes $313.1 million of undiscounted future asset retirement costs estimated as of December 31, 2020, using current estimates of future abandonment costs.