Segment Information (Details) (USD $)	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011		Dec. 31, 2011
Revenues from external customers	$ 685,848,000		$ 767,255,000		$ 1,942,045,000		$ 2,208,756,000
Intersegment revenues	409,563,000		533,028,000		1,086,135,000		1,539,011,000
Operating income (loss)	(220,202,000)	[1]	295,718,000	[1]	(821,883,000)	[1]	810,911,000	[1]
Other Income (Loss), Net	238,000		(122,000)		2,615,000		321,000
Depreciation, depletion and amortization expense	200,655,000		179,113,000		602,112,000		514,180,000
Impairment of natural gas and oil properties	441,465,000				1,377,364,000
Interest expense	10,606,000	[2]	5,657,000	[2]	26,305,000	[2]	19,263,000	[2]
Provision (benefit) for income taxes	(85,755,000)	[2]	114,766,000	[2]	(320,362,000)	[2]	312,733,000	[2]
Assets	7,072,951,000		7,214,107,000		7,072,951,000		7,214,107,000		7,902,897,000
Capital investments	424,886,000	[3]	470,435,000	[3]	1,586,631,000	[3]	1,556,938,000	[3]
Change in accrued expenditures	56,200,000		60,900,000		40,700,000		3,000,000
Intersegment revenues for marketing company's E&P sales	332,100,000		459,400,000		863,700,000		1,327,300,000
Capital investment related to companys E&P segment related to Canada for September 30, 2012 and 2011	2,200,000		8,300,000		6,900,000		16,100,000
Assets related to E&P program in Canada	36,400,000		25,600,000		36,400,000		25,600,000
Non-cash ceiling test impairment	441,500,000				1,377,400,000
Exploration and Production [Member]
Revenues from external customers	493,204,000		553,913,000		1,390,156,000		1,551,538,000
Intersegment revenues	(981,000)		1,707,000		(1,716,000)		10,120,000
Operating income (loss)	(296,108,000)	[1]	228,476,000	[1]	(1,039,737,000)	[1]	629,298,000	[1]
Other Income (Loss), Net	213,000		(17,000)		(34,000)		332,000
Depreciation, depletion and amortization expense	189,714,000		169,391,000		568,654,000		486,130,000
Impairment of natural gas and oil properties	441,465,000				1,377,364,000
Interest expense	6,707,000	[2]	2,003,000	[2]	14,459,000	[2]	5,706,000	[2]
Provision (benefit) for income taxes	(112,822,000)	[2]	89,811,000	[2]	(399,756,000)	[2]	246,685,000	[2]
Assets	5,570,444,000		5,827,527,000		5,570,444,000		5,827,527,000
Capital investments	385,585,000	[3]	421,182,000	[3]	1,450,569,000	[3]	1,365,434,000	[3]
Midstream Services [Member]
Revenues from external customers	192,619,000		213,328,000		551,796,000		657,204,000
Intersegment revenues	409,720,000		530,503,000		1,085,392,000		1,526,504,000
Operating income (loss)	75,488,000	[1]	66,837,000	[1]	216,598,000	[1]	180,398,000	[1]
Other Income (Loss), Net	27,000		(109,000)		4,000		(28,000)
Depreciation, depletion and amortization expense	10,620,000		9,414,000		32,499,000		27,170,000
Impairment of natural gas and oil properties
Interest expense	3,659,000	[2]	3,654,000	[2]	10,904,000	[2]	13,557,000	[2]
Provision (benefit) for income taxes	27,006,000	[2]	24,791,000	[2]	78,268,000	[2]	65,560,000	[2]
Assets	1,158,638,000		1,116,333,000		1,158,638,000		1,116,333,000
Capital investments	31,693,000	[3]	32,158,000	[3]	105,576,000	[3]	137,998,000	[3]
Other [Member]
Revenues from external customers	25,000		14,000		93,000		14,000
Intersegment revenues	824,000		818,000		2,459,000		2,387,000
Operating income (loss)	418,000	[1]	405,000	[1]	1,256,000	[1]	1,215,000	[1]
Other Income (Loss), Net	(2,000)		4,000		2,645,000		17,000
Depreciation, depletion and amortization expense	321,000		308,000		959,000		880,000
Impairment of natural gas and oil properties
Interest expense	240,000	[2]			942,000	[2]
Provision (benefit) for income taxes	61,000	[2]	164,000	[2]	1,126,000	[2]	488,000	[2]
Assets	343,869,000	[4]	270,247,000	[4]	343,869,000	[4]	270,247,000	[4]
Capital investments	$ 7,608,000	[3]	$ 17,095,000	[3]	$ 30,486,000	[3]	$ 53,506,000	[3]

[1] The operating loss for the E&P segment includes a $441.5 million and $1,377.4 million non-cash ceiling test impairment of our natural gas and oil properties for the three and nine-months ended September 30, 2012 respectively.

[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 decreases of $56.2 million and $60.9 million for the three-month periods ended September 30, 2012 and 2011, respectively, and decreases of $40.7 million and $3.0 million for the nine-month periods ended September 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.