Fair Value Measurements - Carrying Amounts and Fair Values of Financial Instruments (Detail) - USD ($) $ in Thousands		Mar. 31, 2016	Dec. 31, 2015
Carrying Value
Assets:
Commodity swaps and basis swaps		$ 266,509	$ 288,762
Marketable securities	[1]	63,018	62,376
(Liabilities):
Commodity swaps and basis swaps		(1,462)	(1,157)
Bank credit facility	[2]	(31,000)	(95,000)
Deferred compensation plan	[3]	(139,279)	(122,918)
Carrying Value | 4.875% Senior Notes Due 2025
(Liabilities):
Subordinated debt	[2]	(750,000)	(750,000)
Carrying Value | 5.75% Senior Subordinated Notes Due 2021
(Liabilities):
Subordinated debt	[2]	(500,000)	(500,000)
Carrying Value | 5.00% Senior Subordinated Notes Due 2022
(Liabilities):
Subordinated debt	[2]	(600,000)	(600,000)
Carrying Value | 5.00% Senior Subordinated Notes Due 2023
(Liabilities):
Subordinated debt	[2]	(750,000)	(750,000)
Fair Value
Assets:
Commodity swaps and basis swaps		266,509	288,762
Marketable securities	[1]	63,018	62,376
(Liabilities):
Commodity swaps and basis swaps		(1,462)	(1,157)
Bank credit facility	[2]	(31,000)	(95,000)
Deferred compensation plan	[3]	(139,279)	(122,918)
Fair Value | 4.875% Senior Notes Due 2025
(Liabilities):
Subordinated debt	[2]	(651,563)	(572,813)
Fair Value | 5.75% Senior Subordinated Notes Due 2021
(Liabilities):
Subordinated debt	[2]	(441,250)	(396,250)
Fair Value | 5.00% Senior Subordinated Notes Due 2022
(Liabilities):
Subordinated debt	[2]	(515,250)	(447,000)
Fair Value | 5.00% Senior Subordinated Notes Due 2023
(Liabilities):
Subordinated debt	[2]	$ (635,625)	$ (551,250)

[1] Marketable securities, which are held in our deferred compensation plans, are actively traded on major exchanges.

[2] The book value of our bank debt approximates fair value because of its floating rate structure. The fair value of our senior notes and our senior subordinated notes is based on end of period market quotes which are Level 2 inputs.

[3] The fair value of our deferred compensation plan is updated at the closing price on the balance sheet date which is a Level 1 input.