Fair Value Measurements - Carrying Amounts and Fair Values of Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Assets
Commodity swaps, collars, call and put options	$ 152,489		$ 251,500
Liabilities
Derivative Liabilities - Commodity swaps, collars, call and put options	13,233		173
Bank debt	(461,000)		(187,000)
Subordinated debt	(2,388,869)		(1,787,967)
7.50% Senior Subordinated Notes due 2017 [Member]
Liabilities
Interest rate on senior subordinated notes	7.50%
7.25% Senior Subordinated notes due 2018 [Member]
Liabilities
Subordinated debt	(250,000)		(250,000)
Interest rate on senior subordinated notes	7.25%		7.25%
8.00% Senior Subordinated notes due 2019 [Member]
Liabilities
Subordinated debt	(288,869)		(287,967)
Interest rate on senior subordinated notes	8.00%		8.00%
6.75% Senior Subordinated notes due 2020 [Member]
Liabilities
Subordinated debt	(500,000)		(500,000)
Interest rate on senior subordinated notes	6.75%		6.75%
5.75% Senior Subordinated Notes due 2021 [Member]
Liabilities
Interest rate on senior subordinated notes	5.75%
5.00% Senior Subordinated notes due 2022 [Member]
Liabilities
Subordinated debt	(600,000)
Interest rate on senior subordinated notes	5.00%		5.00%
Carrying Value [Member]
Assets
Commodity swaps, collars, call and put options	152,489		251,500
Marketable securities	67,429	[1]	50,237	[1]
Liabilities
Derivative Liabilities - Commodity swaps, collars, call and put options	(13,233)		(173)
Bank debt	(461,000)	[2]	(187,000)	[2]
Carrying Value [Member] | 7.50% Senior Subordinated Notes due 2017 [Member]
Liabilities
Subordinated debt	(250,000)	[2]	(250,000)	[2]
Carrying Value [Member] | 7.25% Senior Subordinated notes due 2018 [Member]
Liabilities
Subordinated debt	(250,000)	[2]	(250,000)	[2]
Carrying Value [Member] | 8.00% Senior Subordinated notes due 2019 [Member]
Liabilities
Subordinated debt	(288,869)	[2]	(287,967)	[2]
Carrying Value [Member] | 6.75% Senior Subordinated notes due 2020 [Member]
Liabilities
Subordinated debt	(500,000)	[2]	(500,000)	[2]
Carrying Value [Member] | 5.75% Senior Subordinated Notes due 2021 [Member]
Liabilities
Subordinated debt	(500,000)	[2]	(500,000)	[2]
Carrying Value [Member] | 5.00% Senior Subordinated notes due 2022 [Member]
Liabilities
Subordinated debt	(600,000)	[2]
Fair Value [Member]
Assets
Commodity swaps, collars, call and put options	152,489		251,500
Marketable securities	67,429	[1]	50,237	[1]
Liabilities
Derivative Liabilities - Commodity swaps, collars, call and put options	(13,233)		(173)
Bank debt	(461,000)	[2]	(187,000)	[2]
Fair Value [Member] | 7.50% Senior Subordinated Notes due 2017 [Member]
Liabilities
Subordinated debt	(260,000)	[2]	(265,625)	[2]
Fair Value [Member] | 7.25% Senior Subordinated notes due 2018 [Member]
Liabilities
Subordinated debt	(264,375)	[2]	(267,500)	[2]
Fair Value [Member] | 8.00% Senior Subordinated notes due 2019 [Member]
Liabilities
Subordinated debt	(333,000)	[2]	(334,500)	[2]
Fair Value [Member] | 6.75% Senior Subordinated notes due 2020 [Member]
Liabilities
Subordinated debt	(550,000)	[2]	(555,000)	[2]
Fair Value [Member] | 5.75% Senior Subordinated Notes due 2021 [Member]
Liabilities
Subordinated debt	(537,500)	[2]	(541,250)	[2]
Fair Value [Member] | 5.00% Senior Subordinated notes due 2022 [Member]
Liabilities
Subordinated debt	$ (633,000)	[2]

[1] Marketable securities, which are held in our deferred compensation plans, and 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 subordinated notes is based on end of period market quotes which are Level 2 market values.