Segment Information (Details) (USD $)	3 Months Ended				6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011		Dec. 31, 2011
Revenues from external customers	$ 599,728,000		$ 765,166,000		$ 1,256,197,000		$ 1,441,501,000
Intersegment revenues	318,759,000		526,104,000		676,572,000		1,005,983,000
Operating income (loss)	(652,394,000)	[1]	282,542,000		(466,434,000)	[1]	515,193,000
Other income (loss), net	2,577,000		69,000		2,377,000		443,000
Depreciation, depletion and amortization expense	207,830,000		171,620,000		401,457,000		335,067,000
Impairment of natural gas and oil properties	800,652,000				800,652,000
Interest expense	8,361,000	[2]	6,170,000	[2]	15,699,000	[2]	13,606,000	[2]
Provision for income taxes	(253,046,000)	[2]	108,987,000	[2]	(182,328,000)	[2]	197,967,000	[2]
Assets	7,639,210,000		6,742,101,000		7,639,210,000		6,742,101,000		7,902,897,000
Capital investments	588,633,000	[3]	555,974,000	[3]	1,161,745,000	[3]	1,086,503,000	[3]
Change in accrued expenditures	200,000		56,400,000		15,500,000		57,900,000
Intersegment revenues for marketing company's E&P sales	245,500,000		456,700,000		531,600,000		867,900,000
Capital investment related to companys E&P segment related to Canada for June 30, 2012 and 2011	2,300,000		5,400,000		4,700,000		7,800,000
Assets related to E&P program in Canada	33,200,000		18,400,000		33,200,000		18,400,000
Exploration and Production [Member]
Revenues from external customers	430,687,000		526,969,000		896,952,000		997,625,000
Intersegment revenues	(1,448,000)		2,899,000		(735,000)		8,413,000
Operating income (loss)	(724,625,000)	[1]	222,539,000		(608,382,000)	[1]	400,822,000
Other income (loss), net	(64,000)		6,000		(247,000)		349,000
Depreciation, depletion and amortization expense	196,201,000		161,929,000		378,940,000		316,739,000
Impairment of natural gas and oil properties	800,652,000				800,700,000
Interest expense	4,430,000	[2]	799,000	[2]	7,752,000	[2]	3,703,000	[2]
Provision for income taxes	(279,493,000)	[2]	87,492,000	[2]	(234,655,000)	[2]	156,874,000	[2]
Assets	6,120,697,000		5,366,199,000		6,120,697,000		5,366,199,000
Capital investments	531,845,000	[3]	476,040,000	[3]	1,064,984,000	[3]	944,252,000	[3]
Midstream Services [Member]
Revenues from external customers	169,004,000		238,197,000		359,177,000		443,876,000
Intersegment revenues	319,391,000		522,412,000		675,672,000		996,001,000
Operating income (loss)	71,821,000	[1]	59,644,000		141,110,000	[1]	113,561,000
Other income (loss), net	(5,000)		52,000		(23,000)		81,000
Depreciation, depletion and amortization expense	11,309,000		9,365,000		21,879,000		17,756,000
Impairment of natural gas and oil properties
Interest expense	3,578,000	[2]	5,371,000	[2]	7,245,000	[2]	9,903,000	[2]
Provision for income taxes	25,414,000	[2]	21,349,000	[2]	51,262,000	[2]	40,769,000	[2]
Assets	1,114,364,000		1,095,685,000		1,114,364,000		1,095,685,000
Capital investments	47,719,000	[3]	59,862,000	[3]	73,883,000	[3]	105,840,000	[3]
Other [Member]
Revenues from external customers	37,000				68,000
Intersegment revenues	816,000		793,000		1,635,000		1,569,000
Operating income (loss)	410,000	[1]	359,000		838,000	[1]	810,000
Other income (loss), net	2,646,000		11,000		2,647,000		13,000
Depreciation, depletion and amortization expense	320,000		326,000		638,000		572,000
Impairment of natural gas and oil properties
Interest expense	353,000	[2]			702,000	[2]
Provision for income taxes	1,033,000	[2]	146,000	[2]	1,065,000	[2]	324,000	[2]
Assets	404,149,000	[4]	280,217,000	[4]	404,149,000	[4]	280,217,000	[4]
Capital investments	$ 9,069,000	[3]	$ 20,072,000	[3]	$ 22,878,000	[3]	$ 36,411,000	[3]

[1] The operating loss for the E&P segment for the three and six- months ended June 30, 2012 includes a $800.7 million non-cash ceiling test impairment of our natural gas and oil properties.

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

[3] Capital investments include an increase of $0.2 million and a reduction of $56.4 million for the three-month periods ended June 30, 2012 and 2011, respectively, and increases of $15.5 million and $57.9 million for the six-month periods ended June 30, 2012 and 2011, respectively, relating to the change in accrued expenditures between periods.

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