Capitalized Costs and Accumulated Depreciation, Depletion and Amortization (Detail) - USD ($) $ in Thousands		Apr. 30, 2021		Mar. 31, 2021	Dec. 31, 2020
Natural gas properties:
Properties subject to depletion	[1]	$ 8,991,165			$ 8,891,348
Unproved properties	[1]	862,741			859,766
Total		9,853,906	[1]	$ 9,853,906	9,751,114	[1]
Accumulated depletion and depreciation		(4,150,811)	[1]	(4,150,811)	(4,064,305)	[1]
Natural gas and oil properties, successful efforts method, net		$ 5,703,095	[1]	$ 5,703,095	$ 5,686,809	[1]

[1] Includes capitalized asset retirement costs and the associated accumulated amortization.