Fair Value Measurements (Details 2) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Assets:
Commodity swaps, collars, call options and basis swaps	$ 65,517	$ 131,450
Liabilities:
Commodity swaps, collars, call options and basis swaps	(4,427)	(13,764)
Bank debt	0	274,000
Subordinated debt	1,787,398	1,686,536
6.375% Senior Subordinated Notes due 2015 [Member] | Carrying Value [Member]
Liabilities:
Subordinated debt	0	(150,000)
7.5% senior subordinated notes due 2016, net of discount [Member] | Carrying Value [Member]
Liabilities:
Subordinated debt	0	(249,683)
7.5% Senior Subordinated Notes due 2017 [Member] | Carrying Value [Member]
Liabilities:
Subordinated debt	(250,000)	(250,000)
7.25% Senior Subordinated Notes due 2018 [Member] | Carrying Value [Member]
Liabilities:
Subordinated debt	(250,000)	(250,000)
8.0% senior subordinated notes due 2019, net of discount [Member] | Carrying Value [Member]
Liabilities:
Subordinated debt	(287,398)	(286,853)
6.75% Senior Subordinated Notes due 2020 [Member] | Carrying Value [Member]
Liabilities:
Subordinated debt	(500,000)	(500,000)
Carrying Value [Member]
Assets:
Commodity swaps, collars, call options and basis swaps	65,517	123,255
Commodity collars - discontinued operations	0	8,195
Marketable securities	52,129	47,794
Liabilities:
Commodity swaps, collars, call options and basis swaps	(4,427)	(13,764)
Carrying Value [Member] | Bank credit facility [Member]
Liabilities:
Bank debt	0	(274,000)
Carrying Value [Member] | 5.75% Senior Subordinated Notes due 2021 [Member]
Liabilities:
Subordinated debt	(500,000)	0
6.375% Senior Subordinated Notes due 2015 [Member] | Fair Value [Member]
Liabilities:
Subordinated debt	0	(153,000)
7.5% senior subordinated notes due 2016, net of discount [Member] | Fair Value [Member]
Liabilities:
Subordinated debt	0	(259,375)
7.5% Senior Subordinated Notes due 2017 [Member] | Fair Value [Member]
Liabilities:
Subordinated debt	(265,625)	(263,438)
7.25% Senior Subordinated Notes due 2018 [Member] | Fair Value [Member]
Liabilities:
Subordinated debt	(265,000)	(263,750)
8.0% senior subordinated notes due 2019, net of discount [Member] | Fair Value [Member]
Liabilities:
Subordinated debt	(325,500)	(326,625)
6.75% Senior Subordinated Notes due 2020 [Member] | Fair Value [Member]
Liabilities:
Subordinated debt	(517,500)	(515,625)
Fair Value [Member]
Assets:
Commodity swaps, collars, call options and basis swaps	65,517	123,255
Commodity collars - discontinued operations	0	8,195
Marketable securities	52,129	47,794
Liabilities:
Commodity swaps, collars, call options and basis swaps	(4,427)	(13,764)
Fair Value [Member] | Bank credit facility [Member]
Liabilities:
Bank debt	0	(274,000)
Fair Value [Member] | 5.75% Senior Subordinated Notes due 2021 [Member]
Liabilities:
Subordinated debt	(491,250)	0
6.375% Senior Subordinated Notes due 2015 [Member]
Liabilities:
Subordinated debt	0	150,000
7.5% senior subordinated notes due 2016, net of discount [Member]
Liabilities:
Subordinated debt	0	249,683
7.5% Senior Subordinated Notes due 2017 [Member]
Liabilities:
Subordinated debt	250,000	250,000
7.25% Senior Subordinated Notes due 2018 [Member]
Liabilities:
Subordinated debt	250,000	250,000
8.0% senior subordinated notes due 2019, net of discount [Member]
Liabilities:
Subordinated debt	287,398	286,853
6.75% Senior Subordinated Notes due 2020 [Member]
Liabilities:
Subordinated debt	$ 500,000	$ 500,000