Segment Information (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009		May 25, 2011
Revenues from external customers	$ 744,150,000	$ 767,255,000	$ 765,166,000	$ 676,335,000	$ 670,431,000	$ 682,172,000	$ 589,943,000	$ 668,117,000	$ 2,952,906,000		$ 2,610,663,000		$ 2,145,779,000
Intersegment revenues									2,010,369,000		1,734,605,000		1,051,471,000
Operating income	263,890,000	295,718,000	282,542,000	232,651,000	256,685,000	270,136,000	206,317,000	288,090,000	1,074,801,000		1,021,228,000		(34,966,000)
Other income (loss), net									264,000	[1]	427,000	[1]	1,449,000	[1]
Depreciation, depletion and amortization expense									704,511,000		590,332,000		493,658,000
Impairment of natural gas and oil properties									0		0		907,812,000
Interest expense									24,075,000	[1]	26,163,000	[1]	18,638,000	[1]
Provision for income taxes									413,221,000	[1]	391,659,000	[1]	(16,363,000)
Assets	7,902,897,000				6,017,463,000				7,902,897,000		6,017,463,000		4,770,250,000
Capital investments									2,207,174,000		2,120,065,000		1,809,117,000	[2]
Restricted cash															85,000,000
Change in accrued expenditures									4,300,000		14,400,000		12,200,000
Assets related to E&P program in Canada	28,400,000				11,500,000				28,400,000		11,500,000
Exploration and Production [Member]
Revenues from external customers									2,088,763,000		1,871,835,000		1,582,596,000
Intersegment revenues									11,725,000		18,609,000		10,635,000
Operating income									825,138,000		829,462,000		(157,725,000)
Other income (loss), net									328,000	[1]	235,000	[1]	1,406,000	[1]
Depreciation, depletion and amortization expense									666,125,000		561,018,000		474,014,000
Impairment of natural gas and oil properties													907,812,000
Interest expense									9,026,000	[1]	7,888,000	[1]	15,237,000	[1]
Provision for income taxes									322,714,000	[1]	323,748,000	[1]	(61,725,000)
Assets	6,547,117,000				4,849,478,000				6,547,117,000		4,849,478,000		3,904,739,000
Capital investments									1,977,493,000		1,775,518,000		1,565,450,000	[2]
Midstream Services [Member]
Revenues from external customers									864,096,000		738,828,000		562,944,000
Intersegment revenues									1,995,423,000		1,715,012,000		1,040,388,000
Operating income									247,952,000		191,566,000		122,620,000
Other income (loss), net									(91,000)	[1]	179,000	[1]	34,000	[1]
Depreciation, depletion and amortization expense									37,261,000		28,765,000		19,213,000
Impairment of natural gas and oil properties													0
Interest expense									15,049,000	[1]	18,275,000	[1]	3,401,000	[1]
Provision for income taxes									90,221,000	[1]	67,834,000	[1]	45,303,000
Assets	1,119,861,000				1,016,563,000				1,119,861,000		1,016,563,000		767,346,000
Capital investments									160,776,000		271,316,000		214,208,000	[2]
Other [Member]
Revenues from external customers									47,000		0		239,000
Intersegment revenues									3,221,000		984,000		448,000
Operating income									1,711,000		200,000		139,000
Other income (loss), net									27,000	[1]	13,000	[1]	9,000	[1]
Depreciation, depletion and amortization expense									1,125,000		549,000		431,000
Impairment of natural gas and oil properties													0
Interest expense									0	[1]	0	[1]	0	[1]
Provision for income taxes									286,000	[1]	77,000	[1]	59,000
Assets	235,919,000				151,422,000				235,919,000		151,422,000		98,165,000
Capital investments									$ 68,905,000		$ 73,231,000		$ 29,459,000	[2]

[1] (1) Interest income, interest expense and the provision (benefit) for income taxes by segment are an allocation of corporate amounts as cash equivalents, debt and income tax expense allocated as they are incurred at the corporate level.

[2] (3) Capital investments include an increase of $4.3 million for 2011, an increase of $14.4 million for 2010 and an increase of $12.2 million for 2009 related to the change in accrued expenditures between years.