Fair Value Measurements - Carrying Amounts and Fair Values of Financial Instruments (Detail) - USD ($) $ in Thousands		Dec. 31, 2015	Dec. 31, 2014
Carrying Value
Assets:
Commodity swaps, collars and basis swaps		$ 288,762	$ 403,363
Marketable securities	[1]	62,376	68,454
Liabilities:
Commodity swaps, collars and basis swaps		(1,157)	0
Bank credit facility	[2]	(95,000)	(723,000)
Deferred compensation plan	[3]	(122,918)	(203,433)
Carrying Value | 4.875% Senior Notes Due 2025
Liabilities:
Subordinated debt	[2]	(750,000)	0
Carrying Value | 6.75% Senior Subordinated Notes Due 2020
Liabilities:
Subordinated debt	[2]	0	(500,000)
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, collars and basis swaps		288,762	403,363
Marketable securities	[1]	62,376	68,454
Liabilities:
Commodity swaps, collars and basis swaps		(1,157)	0
Bank credit facility	[2]	(95,000)	(723,000)
Deferred compensation plan	[3]	(122,918)	(203,433)
Fair Value | 4.875% Senior Notes Due 2025
Liabilities:
Subordinated debt	[2]	(572,813)	0
Fair Value | 6.75% Senior Subordinated Notes Due 2020
Liabilities:
Subordinated debt	[2]	0	(523,125)
Fair Value | 5.75% Senior Subordinated Notes Due 2021
Liabilities:
Subordinated debt	[2]	(396,250)	(520,000)
Fair Value | 5.00% Senior Subordinated Notes Due 2022
Liabilities:
Subordinated debt	[2]	(447,000)	(601,500)
Fair Value | 5.00% Senior Subordinated Notes Due 2023
Liabilities:
Subordinated debt	[2]	$ (551,250)	$ (754,688)

[1] Marketable securities are held in our deferred compensation plans that 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 based on closing prices on the balance sheet date.