Fair Value Measurements - Carrying Amounts and Fair Values of Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014		Dec. 31, 2013
(Liabilities):
Bank credit facility	$ (480,000)		$ (500,000)
Subordinated debt	(2,350,000)		(2,640,516)
8.00% Senior Subordinated notes due 2019
(Liabilities):
Subordinated debt			(290,516)
6.75% Senior Subordinated notes due 2020
(Liabilities):
Subordinated debt	(500,000)		(500,000)
5.75% Senior Subordinated notes due 2021
(Liabilities):
Subordinated debt	(500,000)		(500,000)
5.00% Senior Subordinated notes due 2022
(Liabilities):
Subordinated debt	(600,000)		(600,000)
5.00% Senior Subordinated notes due 2023
(Liabilities):
Subordinated debt	(750,000)		(750,000)
Carrying Value
Assets:
Commodity swaps, collars and basis swaps	10,223		13,654
Marketable securities	68,129	[1]	67,766	[1]
(Liabilities):
Commodity swaps, collars and basis swaps	(70,066)		(26,223)
Bank credit facility	(480,000)	[2]	(500,000)	[2]
Deferred compensation plan	(276,868)	[3]	(271,738)	[3]
Carrying Value | 8.00% Senior Subordinated notes due 2019
(Liabilities):
Subordinated debt			(290,516)	[2]
Carrying Value | 6.75% Senior Subordinated notes due 2020
(Liabilities):
Subordinated debt	(500,000)	[2]	(500,000)	[2]
Carrying Value | 5.75% Senior Subordinated notes due 2021
(Liabilities):
Subordinated debt	(500,000)	[2]	(500,000)	[2]
Carrying Value | 5.00% Senior Subordinated notes due 2022
(Liabilities):
Subordinated debt	(600,000)	[2]	(600,000)	[2]
Carrying Value | 5.00% Senior Subordinated notes due 2023
(Liabilities):
Subordinated debt	(750,000)	[2]	(750,000)	[2]
Fair Value
Assets:
Commodity swaps, collars and basis swaps	10,223		13,654
Marketable securities	68,129	[1]	67,766	[1]
(Liabilities):
Commodity swaps, collars and basis swaps	(70,066)		(26,223)
Bank credit facility	(480,000)	[2]	(500,000)	[2]
Deferred compensation plan	(276,868)	[3]	(271,738)	[3]
Fair Value | 8.00% Senior Subordinated notes due 2019
(Liabilities):
Subordinated debt			(319,500)	[2]
Fair Value | 6.75% Senior Subordinated notes due 2020
(Liabilities):
Subordinated debt	(539,375)	[2]	(541,250)	[2]
Fair Value | 5.75% Senior Subordinated notes due 2021
(Liabilities):
Subordinated debt	(540,000)	[2]	(530,625)	[2]
Fair Value | 5.00% Senior Subordinated notes due 2022
(Liabilities):
Subordinated debt	(636,000)	[2]	(588,750)	[2]
Fair Value | 5.00% Senior Subordinated notes due 2023
(Liabilities):
Subordinated debt	$ (798,750)	[2]	$ (732,188)	[2]

[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 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 on the closing price on the balance sheet date which is a Level 1 input.