Segment Information (Details) (USD $)	3 Months Ended				6 Months Ended
	Jun. 30, 2011		Jun. 30, 2010		Jun. 30, 2011		Jun. 30, 2010		May 25, 2011	Dec. 31, 2010
Revenues from external customers	$ 765,166,000		$ 589,943,000		$ 1,441,501,000		$ 1,258,060,000
Intersegment revenues	526,104,000		391,016,000		1,005,983,000		846,857,000
Operating income	282,542,000		206,317,000		515,193,000		494,407,000
Other income, net	69,000	[1]	(84,000)	[1]	443,000	[1]	(61,000)	[1]
Depreciation, depletion and amortization expense	171,620,000		144,006,000		335,067,000		283,023,000
Interest expense	6,170,000	[1]	6,180,000	[1]	13,606,000	[1]	12,688,000	[1]
Provision for income taxes	108,987,000	[1]	78,044,000	[1]	197,967,000	[1]	187,881,000	[1]
Assets	6,742,101,000		5,590,351,000		6,742,101,000		5,590,351,000			6,017,463,000
Capital investments	555,974,000	[2]	543,525,000	[2]	1,086,503,000	[2]	1,017,150,000	[2]
Restricted cash									85,000,000
Increase in accrued expenditures between periods	56,400,000		2,300,000		57,900,000		25,000,000
Intersegment revenues for marketing company's E&P sales	456,700,000		335,100,000		867,900,000		741,000,000
Capital investment related to companys E&P segment related to Canada	5,400,000		7,300,000		7,800,000		7,300,000
Assets related to E&P program in Canada	18,400,000		7,300,000		18,400,000		7,300,000
Exploration and Production [Member]
Revenues from external customers	526,969,000		419,149,000		997,625,000		904,220,000
Intersegment revenues	2,899,000		2,706,000		8,413,000		9,704,000
Operating income	222,539,000		162,473,000		400,822,000		412,904,000
Other income, net	6,000	[1]	(107,000)	[1]	349,000	[1]	(152,000)	[1]
Depreciation, depletion and amortization expense	161,929,000		136,905,000		316,739,000		269,612,000
Interest expense	799,000	[1]	704,000	[1]	3,703,000	[1]	3,004,000	[1]
Provision for income taxes	87,492,000	[1]	63,072,000	[1]	156,874,000	[1]	159,837,000	[1]
Assets	5,366,199,000	[3]	4,272,197,000	[3]	5,366,199,000	[3]	4,272,197,000	[3]
Capital investments	476,040,000	[2]	441,226,000	[2]	944,252,000	[2]	852,659,000	[2]
Midstream Services [Member]
Revenues from external customers	238,197,000		170,794,000		443,876,000		353,840,000
Intersegment revenues	522,412,000		388,064,000		996,001,000		836,661,000
Operating income	59,644,000		43,767,000		113,561,000		81,391,000
Other income, net	52,000	[1]	11,000	[1]	81,000	[1]	79,000	[1]
Depreciation, depletion and amortization expense	9,365,000		6,986,000		17,756,000		13,147,000
Interest expense	5,371,000	[1]	5,476,000	[1]	9,903,000	[1]	9,684,000	[1]
Provision for income taxes	21,349,000	[1]	14,938,000	[1]	40,769,000	[1]	27,997,000	[1]
Assets	1,095,685,000		839,959,000		1,095,685,000		839,959,000
Capital investments	59,862,000	[2]	89,753,000	[2]	105,840,000	[2]	139,019,000	[2]
Other [Member]
Revenues from external customers	0		0		0		0
Intersegment revenues	793,000		246,000		1,569,000		492,000
Operating income	359,000		77,000		810,000		112,000
Other income, net	11,000	[1]	12,000	[1]	13,000	[1]	12,000	[1]
Depreciation, depletion and amortization expense	326,000		115,000		572,000		264,000
Interest expense	0	[1]	0	[1]	0	[1]	0	[1]
Provision for income taxes	146,000	[1]	34,000	[1]	324,000	[1]	47,000	[1]
Assets	280,217,000	[4]	478,195,000	[4]	280,217,000	[4]	478,195,000	[4]
Capital investments	$ 20,072,000	[2]	$ 12,546,000	[2]	$ 36,411,000	[2]	$ 25,472,000	[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 an increase of $56.4 million and a reduction of $2.3 million for the three-month periods ended June 30, 2011 and 2010, respectively, and increases of $57.9 million and $25.0 million for the six-month periods ended June 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 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.