Segment Information (Details) (USD $)	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012		Dec. 31, 2012
Revenues from external customers	$ 868,366,000		$ 691,727,000		$ 2,464,056,000		$ 1,952,638,000
Intersegment revenues	601,812,000		409,563,000		1,744,126,000		1,086,135,000
Operating income (loss)	309,229,000		(65,959,000)		886,506,000		(527,679,000)	[1]
Other income (loss), net	(319,000)		238,000	[1]	(498,000)	[2]	2,615,000	[2]
Gain (loss) on Derivatives	12,124,000	[2]	(5,879,000)	[2]	75,779,000		(10,593,000)
Depreciation, depletion and amortization expense	204,934,000		203,935,000		571,268,000		605,392,000
Impairment of natural gas and oil properties			289,821,000				1,090,473,000	[2]
Interest expense	11,018,000		10,606,000		29,317,000	[2]	26,305,000	[2]
Provision (benefit) for income taxes	124,149,000	[2]	(28,153,000)	[2]	373,457,000	[2]	(210,481,000)	[2]
Assets	7,894,388,000	[2]	7,356,562,000	[2],[3]	7,894,388,000	[2]	7,356,562,000	[2],[3]	6,737,527,000
Capital investments	542,109,000		424,886,000		1,755,058,000	[3]	1,586,631,000	[3]
Change in accrued expenditures	14,600,000		56,200,000		25,800,000		40,700,000
Intersegment revenues for marketing company's E&P sales	515,300,000		332,100,000		1,493,800,000		863,700,000
Capital investment related to companys E&P segment related to Canada for September 30, 2012 and 2011	11,900,000		2,200,000		19,800,000		6,900,000
Assets related to E&P program in Canada	64,800,000		36,400,000		64,800,000		36,400,000
Exploration and Production [Member]
Revenues from external customers	621,833,000		499,083,000		1,748,821,000		1,400,749,000
Intersegment revenues	644,000		(982,000)		3,817,000		(1,716,000)
Operating income (loss)	222,692,000		(141,865,000)		650,996,000		(745,533,000)	[1]
Other income (loss), net	143,000		213,000	[1]	61,000	[2]	(34,000)	[2]
Gain (loss) on Derivatives	10,885,000	[2]	(5,879,000)	[2]	74,540,000		(10,593,000)
Depreciation, depletion and amortization expense	191,860,000		192,994,000		533,577,000		571,934,000
Impairment of natural gas and oil properties			289,821,000				1,090,473,000	[2]
Interest expense	8,580,000		6,707,000		21,101,000	[2]	14,459,000	[2]
Provision (benefit) for income taxes	87,779,000	[2]	(55,220,000)	[2]	282,231,000	[2]	(289,875,000)	[2]
Assets	6,264,845,000	[2]	5,854,055,000	[2],[3]	6,264,845,000	[2]	5,854,055,000	[2],[3]
Capital investments	496,331,000		385,585,000		1,602,885,000	[3]	1,450,569,000	[3]
Midstream Services [Member]
Revenues from external customers	246,501,000		192,619,000		715,155,000		551,796,000
Intersegment revenues	601,104,000		409,720,000		1,740,132,000		1,085,392,000
Operating income (loss)	86,658,000		75,488,000		235,854,000		216,598,000	[1]
Other income (loss), net	70,000		27,000	[1]	(27,000)	[2]	4,000	[2]
Gain (loss) on Derivatives	1,232,000	[2]			1,232,000
Depreciation, depletion and amortization expense	12,970,000		10,620,000		37,377,000		32,499,000
Interest expense	2,294,000		3,659,000		7,682,000	[2]	10,904,000	[2]
Provision (benefit) for income taxes	36,671,000	[2]	27,006,000	[2]	91,785,000	[2]	78,268,000	[2]
Assets	1,387,765,000	[2]	1,158,638,000	[2],[3]	1,387,765,000	[2]	1,158,638,000	[2],[3]
Capital investments	40,052,000		31,693,000		135,425,000	[3]	105,576,000	[3]
Other [Member]
Revenues from external customers	32,000		25,000		80,000		93,000
Intersegment revenues	64,000		825,000		177,000		2,459,000
Operating income (loss)	(121,000)		418,000		(344,000)		1,256,000	[1]
Other income (loss), net	(532,000)		(2,000)	[1]	(532,000)	[2]	2,645,000	[2]
Gain (loss) on Derivatives	7,000	[2]			7,000
Depreciation, depletion and amortization expense	104,000		321,000		314,000		959,000
Interest expense	144,000		240,000		534,000	[2]	942,000	[2],[4]
Provision (benefit) for income taxes	(301,000)	[2]	61,000	[2]	(559,000)	[2]	1,126,000	[2],[4]
Assets	241,778,000	[2],[4]	343,869,000	[2],[3]	241,778,000	[2],[4]	343,869,000	[2],[3]
Capital investments	5,726,000		7,608,000		16,748,000	[3]	30,486,000	[3]
Parent [Member]
Assets	$ 5,538,128,000				$ 5,538,128,000				$ 4,809,628,000

[1] The operating loss for the E&P segment for the three and nine months ended September 30, 2012 includes a $289.8 million and $1,090.5 million non-cash ceiling test impairment of our natural gas and oil properties respectively.

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

[3] Capital investments includes decreases of $14.6 million and $56.2 million for the three-month periods ended September 30, 2013 and 2012, respectively, and increase of $25.8 million and decrease of $40.7 million for the nine-month periods ended September 30, 2013 and 2012, 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.