Segment Information (Details) (USD $)	3 Months Ended				9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010		Sep. 30, 2011		Sep. 30, 2010		May 25, 2011	Dec. 31, 2010
Revenues from external customers	$ 767,255,000		$ 682,172,000		$ 2,208,756,000		$ 1,940,232,000
Intersegment revenues	533,028,000		456,884,000		1,539,011,000		1,303,741,000
Operating income	295,718,000		270,136,000		810,911,000		764,543,000
Other income (loss), net	(122,000)	[1]	326,000	[1]	321,000	[1]	265,000	[1]
Depreciation, depletion and amortization expense	179,113,000		151,284,000		514,180,000		434,307,000
Interest expense	5,657,000	[1]	6,589,000	[1]	19,263,000	[1]	19,277,000	[1]
Provision for income taxes	114,766,000	[1]	103,235,000	[1]	312,733,000	[1]	291,116,000	[1]
Assets	7,214,107,000		5,992,724,000		7,214,107,000		5,992,724,000			6,017,463,000
Capital investments	470,435,000	[2]	516,630,000	[2]	1,556,938,000	[2]	1,533,780,000	[2]
Restricted cash									85,000,000
Reduction in accrued expenditures between periods	60,900,000		29,800,000		3,000,000		4,800,000
Intersegment revenues for marketing company's E&P sales	459,400,000		394,300,000		1,327,300,000		1,135,300,000
Capital investment related to companys E&P segment related to Canada	8,300,000		2,500,000		16,100,000		9,800,000
Assets related to E&P program in Canada	25,600,000		9,700,000		25,600,000		9,700,000
Exploration and Production [Member]
Revenues from external customers	553,913,000		487,133,000		1,551,538,000		1,391,353,000
Intersegment revenues	1,707,000		4,767,000		10,120,000		14,471,000
Operating income	228,476,000		216,696,000		629,298,000		629,600,000
Other income (loss), net	(17,000)	[1]	219,000	[1]	332,000	[1]	67,000	[1]
Depreciation, depletion and amortization expense	169,391,000		143,457,000		486,130,000		413,069,000
Interest expense	2,003,000	[1]	1,701,000	[1]	5,706,000	[1]	4,705,000	[1]
Provision for income taxes	89,811,000	[1]	84,257,000	[1]	246,685,000	[1]	244,094,000	[1]
Assets	5,827,527,000	[3]	4,572,128,000	[3]	5,827,527,000	[3]	4,572,128,000	[3]
Capital investments	421,182,000	[2]	420,294,000	[2]	1,365,434,000	[2]	1,272,953,000	[2]
Midstream Services [Member]
Revenues from external customers	213,328,000		195,039,000		657,204,000		548,879,000
Intersegment revenues	530,503,000		451,870,000		1,526,504,000		1,288,531,000
Operating income	66,837,000		53,390,000		180,398,000		134,781,000
Other income (loss), net	(109,000)	[1]	107,000	[1]	(28,000)	[1]	186,000	[1]
Depreciation, depletion and amortization expense	9,414,000		7,684,000		27,170,000		20,831,000
Interest expense	3,654,000	[1]	4,888,000	[1]	13,557,000	[1]	14,572,000	[1]
Provision for income taxes	24,791,000	[1]	18,957,000	[1]	65,560,000	[1]	46,954,000	[1]
Assets	1,116,333,000		910,506,000		1,116,333,000		910,506,000
Capital investments	32,158,000	[2]	77,006,000	[2]	137,998,000	[2]	216,025,000	[2]
Other [Member]
Revenues from external customers	14,000		0		14,000		0
Intersegment revenues	818,000		247,000		2,387,000		739,000
Operating income	405,000		50,000		1,215,000		162,000
Other income (loss), net	4,000	[1]	0	[1]	17,000	[1]	12,000	[1]
Depreciation, depletion and amortization expense	308,000		143,000		880,000		407,000
Interest expense	0	[1]	0	[1]	0	[1]	0	[1]
Provision for income taxes	164,000	[1]	21,000	[1]	488,000	[1]	68,000	[1]
Assets	270,247,000	[4]	510,090,000	[4]	270,247,000	[4]	510,090,000	[4]
Capital investments	$ 17,095,000	[2]	$ 19,330,000	[2]	$ 53,506,000	[2]	$ 44,802,000	[2]

[1] (1) Interest income, interest expense and the provision for income taxes by segment are allocated as they are incurred at the corporate level.

[2] (4) Capital investments include reductions of $60.9 million and a reduction of $29.8 million for the three-month periods ended September 30, 2011 and 2010, respectively, and reductions of $3.0 million and $4.8 million for the nine-month periods ended September 30, 2011 and 2010, respectively, relating to the change in accrued expenditures between periods.

[3] (2) Includes capital investments for office, technology, drilling rigs and other ancillary equipment not directly related to natural gas and oil property acquisition, exploration and development activities.

[4] (3) Other assets represent corporate assets not allocated to segments and assets, including restricted cash and investments in cash equivalents, for non-reportable segments.