Fair Value Measurements - Carrying Amounts and Fair Values of Financial Instruments (Detail) - USD ($) $ in Thousands		Sep. 30, 2015	May. 31, 2015	Dec. 31, 2014
(Liabilities):
Senior notes		$ (750,000)		$ 0
4.875% Senior Notes Due 2025
(Liabilities):
Senior notes		(750,000)	$ (750,000)	0
Carrying Value
Assets:
Commodity swaps, collars and basis swaps		333,175		403,363
Marketable securities	[1]	60,273		68,454
(Liabilities):
Basis swaps		(404)		0
Bank credit facility	[2]	(987,000)		(723,000)
Deferred compensation plan	[3]	(137,637)		(203,433)
Carrying Value | 4.875% Senior Notes Due 2025
(Liabilities):
Senior notes	[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		333,175		403,363
Marketable securities	[1]	60,273		68,454
(Liabilities):
Basis swaps		(404)		0
Bank credit facility	[2]	(987,000)		(723,000)
Deferred compensation plan	[3]	(137,637)		(203,433)
Fair Value | 4.875% Senior Notes Due 2025
(Liabilities):
Senior notes	[2]	(666,563)		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]	(471,875)		(520,000)
Fair Value | 5.00% Senior Subordinated Notes Due 2022
(Liabilities):
Subordinated debt	[2]	(532,500)		(601,500)
Fair Value | 5.00% Senior Subordinated Notes Due 2023
(Liabilities):
Subordinated debt	[2]	$ (664,688)		$ (754,688)

[1] Marketable securities, which are held in our deferred compensation plans, are actively traded on major exchanges. Refer to Note 13 for additional information.

[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. Refer to Note 8 for additional information.

[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.