Capitalized Costs and Accumulated Depreciation, Depletion and Amortization (Detail) - USD ($) $ in Thousands		Sep. 30, 2021	Dec. 31, 2020
Natural gas properties:
Properties subject to depletion	[1]	$ 9,203,586	$ 8,891,348
Unproved properties	[1]	864,659	859,766
Total	[1]	10,068,245	9,751,114
Accumulated depletion and depreciation	[1]	(4,330,289)	(4,064,305)
Natural gas and oil properties, successful efforts method, net	[1]	$ 5,737,956	$ 5,686,809

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