Fair Value Measurements - Carrying Amounts and Fair Values of Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Assets
Commodity swaps and collars	$ 27,439		$ 153,267
Liabilities
Commodity swaps and collars	(28,032)		(7,934)
Bank credit facility	(47,000)		(739,000)
Deferred compensation plan	(222,700)		(187,604)
Subordinated debt	(2,889,505)		(2,139,185)
Carrying Value
Assets
Commodity swaps and collars	27,439		153,267
Marketable securities	71,440	[1]	57,776	[1]
Liabilities
Commodity swaps and collars	(28,032)		(7,934)
Bank credit facility	(47,000)	[2]	(739,000)	[2]
Deferred compensation plan	(222,700)	[2]	(187,604)	[2]
Fair Value
Assets
Commodity swaps and collars	27,439		153,267
Marketable securities	71,440	[1]	57,776	[1]
Liabilities
Commodity swaps and collars	(28,032)		(7,934)
Bank credit facility	(47,000)	[2]	(739,000)	[2]
Deferred compensation plan	(222,700)	[2]	(187,604)	[2]
7.25% Senior Subordinated notes due 2018
Liabilities
Subordinated debt	(250,000)		(250,000)
7.25% Senior Subordinated notes due 2018 | Carrying Value
Liabilities
Subordinated debt	(250,000)	[2]	(250,000)	[2]
7.25% Senior Subordinated notes due 2018 | Fair Value
Liabilities
Subordinated debt	(260,000)	[2]	(262,500)	[2]
8.00% Senior Subordinated notes due 2019
Liabilities
Subordinated debt	(289,505)		(289,185)
8.00% Senior Subordinated notes due 2019 | Carrying Value
Liabilities
Subordinated debt	(289,505)	[2]	(289,185)	[2]
8.00% Senior Subordinated notes due 2019 | Fair Value
Liabilities
Subordinated debt	(328,500)	[2]	(332,250)	[2]
6.75% Senior Subordinated notes due 2020
Liabilities
Subordinated debt	(500,000)		(500,000)
6.75% Senior Subordinated notes due 2020 | Carrying Value
Liabilities
Subordinated debt	(500,000)	[2]	(500,000)	[2]
6.75% Senior Subordinated notes due 2020 | Fair Value
Liabilities
Subordinated debt	(550,000)	[2]	(542,500)	[2]
5.75% Senior Subordinated Notes due 2021 | Carrying Value
Liabilities
Subordinated debt	(500,000)	[2]	(500,000)	[2]
5.75% Senior Subordinated Notes due 2021 | Fair Value
Liabilities
Subordinated debt	(536,250)	[2]	(535,000)	[2]
5.00% Senior Subordinated notes due 2022
Liabilities
Subordinated debt	(600,000)		(600,000)
5.00% Senior Subordinated notes due 2022 | Carrying Value
Liabilities
Subordinated debt	(600,000)	[2]	(600,000)	[2]
5.00% Senior Subordinated notes due 2022 | Fair Value
Liabilities
Subordinated debt	(613,500)	[2]	(627,000)	[2]
5.00% senior subordinated notes due 2023 | Carrying Value
Liabilities
Subordinated debt	(750,000)	[2]
5.00% senior subordinated notes due 2023 | Fair Value
Liabilities
Subordinated debt	$ (766,875)	[2]

[1] Marketable securities, which are held in our deferred compensation plans, 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. Refer to Note 8 for additional information.