Capitalized Costs and Accumulated Depreciation, Depletion and Amortization (Detail) - USD ($) $ in Thousands		Sep. 30, 2018	Dec. 31, 2017
Natural gas and oil properties:
Properties subject to depletion	[1]	$ 11,041,412	$ 10,572,453
Unproved properties	[1]	2,601,784	2,644,000
Total	[1]	13,643,196	13,216,453
Accumulated depletion and depreciation	[1]	(3,929,060)	(3,649,716)
Natural gas and oil properties, successful efforts method, net	[1]	$ 9,714,136	$ 9,566,737

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