Fair Value Measurements - Carrying Amounts and Fair Values of Financial Instruments (Detail) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Assets:
Commodity swaps, collars, basis swaps, call and put options	$ 153,267	$ 251,500
Marketable securities	57,800,000	50,200,000
Liabilities:
Commodity swaps, collars and call options	7,934	173
Bank credit facility	(739,000,000)	(187,000,000)
Subordinated debt	(2,139,185,000)	(1,787,967,000)
7.25% Senior Subordinated notes due 2018 [Member]
Liabilities:
Subordinated debt	(250,000,000)	(250,000,000)
8.00% Senior Subordinated notes due 2019 [Member]
Liabilities:
Subordinated debt	(289,185,000)	(287,967,000)
6.75% Senior Subordinated notes due 2020 [Member]
Liabilities:
Subordinated debt	(500,000,000)	(500,000,000)
5.00% Senior Subordinated notes due 2022 [Member]
Liabilities:
Subordinated debt	(600,000,000)
Carrying Value [Member]
Assets:
Commodity swaps, collars, basis swaps, call and put options	153,267,000	251,500,000
Marketable securities	57,776,000	50,237,000
Liabilities:
Commodity swaps, collars and call options	(7,934,000)	(173,000)
Bank credit facility	(739,000,000)	(187,000,000)
Deferred compensation plan	(187,604,000)	(169,188,000)
Carrying Value [Member] | 7.5% Senior Subordinated Notes due 2017 [Member]
Liabilities:
Subordinated debt		(250,000,000)
Carrying Value [Member] | 7.25% Senior Subordinated notes due 2018 [Member]
Liabilities:
Subordinated debt	(250,000,000)	(250,000,000)
Carrying Value [Member] | 8.00% Senior Subordinated notes due 2019 [Member]
Liabilities:
Subordinated debt	(289,185,000)	(287,967,000)
Carrying Value [Member] | 6.75% Senior Subordinated notes due 2020 [Member]
Liabilities:
Subordinated debt	(500,000,000)	(500,000,000)
Carrying Value [Member] | 5.75% Senior Subordinated Notes due 2021 [Member]
Liabilities:
Subordinated debt	(500,000,000)	(500,000,000)
Carrying Value [Member] | 5.00% Senior Subordinated notes due 2022 [Member]
Liabilities:
Subordinated debt	(600,000,000)
Fair Value [Member]
Assets:
Commodity swaps, collars, basis swaps, call and put options	153,267,000	251,500,000
Marketable securities	57,776,000	50,237,000
Liabilities:
Commodity swaps, collars and call options	(7,934,000)	(173,000)
Bank credit facility	(739,000,000)	(187,000,000)
Deferred compensation plan	(187,604,000)	(169,188,000)
Fair Value [Member] | 7.5% Senior Subordinated Notes due 2017 [Member]
Liabilities:
Subordinated debt		(265,625,000)
Fair Value [Member] | 7.25% Senior Subordinated notes due 2018 [Member]
Liabilities:
Subordinated debt	(262,500,000)	(267,500,000)
Fair Value [Member] | 8.00% Senior Subordinated notes due 2019 [Member]
Liabilities:
Subordinated debt	(332,250,000)	(334,500,000)
Fair Value [Member] | 6.75% Senior Subordinated notes due 2020 [Member]
Liabilities:
Subordinated debt	(542,500,000)	(555,000,000)
Fair Value [Member] | 5.75% Senior Subordinated Notes due 2021 [Member]
Liabilities:
Subordinated debt	(535,000,000)	(541,250,000)
Fair Value [Member] | 5.00% Senior Subordinated notes due 2022 [Member]
Liabilities:
Subordinated debt	$ (627,000,000)