Segment Information (Details) (USD $)	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012		Dec. 31, 2012
Revenues from external customers	$ 862,041,000		$ 606,076,000		$ 1,595,690,000		$ 1,260,911,000
Intersegment revenues	643,379,000		318,759,000		1,142,314,000		676,572,000
Operating income (loss)	325,317,000		(646,046,000)	[1]	577,277,000		(461,720,000)	[1]
Other Gain (Loss), Net	354,000	[2]	2,577,000	[2]	(179,000)	[2]	2,377,000	[2]
Gain (Loss) on Derivatives	93,449,000		(6,348,000)		63,655,000		(4,714,000)
Depreciation, depletion and amortization expense	186,867,000		207,830,000		366,334,000		401,457,000
Impairment of natural gas and oil properties			800,652,000				800,652,000	[2]
Interest expense	9,278,000	[2]	8,361,000	[2]	18,299,000	[2]	15,699,000	[2]
Provision (benefit) for income taxes	164,211,000	[2]	(253,046,000)	[2]	249,308,000	[2]	(182,328,000)	[2]
Assets	7,635,191,000		7,639,210,000	[3]	7,635,191,000		7,639,210,000	[3]	6,737,527,000
Capital investments	694,893,000	[3]	588,633,000	[2]	1,212,949,000	[3]	1,161,745,000	[3]
Change in accrued expenditures	7,500,000		200,000		40,400,000		15,500,000
Intersegment revenues for marketing company's E&P sales	559,000,000		245,500,000		978,500,000		531,600,000
Capital investment related to companys E&P segment related to Canada for September 30, 2012 and 2011	5,600,000		2,300,000		7,900,000		4,700,000
Assets related to E&P program in Canada	51,300,000		33,200,000		51,300,000		33,200,000
Non-cash ceiling test impairment			800,700,000				800,700,000
Exploration and Production [Member]
Revenues from external customers	616,894,000		437,035,000		1,126,988,000		901,666,000
Intersegment revenues	1,664,000		(1,448,000)		3,173,000		(735,000)
Operating income (loss)	252,546,000		(718,277,000)	[1]	428,304,000		(603,668,000)	[1]
Other Gain (Loss), Net	296,000	[2]	(64,000)	[2]	(82,000)	[2]	(247,000)	[2]
Gain (Loss) on Derivatives	93,449,000		(6,348,000)		63,655,000		(4,714,000)
Depreciation, depletion and amortization expense	174,267,000		196,201,000		341,717,000		378,940,000
Impairment of natural gas and oil properties			800,652,000				800,652,000	[2]
Interest expense	6,345,000	[2]	4,430,000	[2]	12,521,000	[2]	7,752,000	[2]
Provision (benefit) for income taxes	138,636,000	[2]	(279,493,000)	[2]	194,452,000	[2]	(234,655,000)	[2]
Assets	6,006,318,000		6,120,697,000	[3]	6,006,318,000		6,120,697,000	[3]
Capital investments	631,221,000	[3]	531,845,000	[2]	1,106,554,000	[3]	1,064,984,000	[3]
Midstream Services [Member]
Revenues from external customers	245,120,000		169,004,000		468,654,000		359,177,000
Intersegment revenues	641,655,000		319,391,000		1,139,028,000		675,672,000
Operating income (loss)	72,889,000		71,821,000	[1]	149,196,000		141,110,000	[1]
Other Gain (Loss), Net	58,000	[2]	(5,000)	[2]	(97,000)	[2]	(23,000)	[2]
Depreciation, depletion and amortization expense	12,495,000		11,309,000		24,407,000		21,879,000
Interest expense	2,743,000	[2]	3,578,000	[2]	5,388,000	[2]	7,245,000	[2]
Provision (benefit) for income taxes	25,711,000	[2]	25,414,000	[2]	55,114,000	[2]	51,262,000	[2]
Assets	1,383,930,000		1,114,364,000	[3]	1,383,930,000		1,114,364,000	[3]
Capital investments	56,906,000	[3]	47,719,000	[2]	95,373,000	[3]	73,883,000	[3]
Other [Member]
Revenues from external customers	27,000		37,000		48,000		68,000
Intersegment revenues	60,000		816,000		113,000		1,635,000
Operating income (loss)	(118,000)		410,000	[1]	(223,000)		838,000	[1]
Other Gain (Loss), Net			2,646,000	[2]			2,647,000	[2]
Depreciation, depletion and amortization expense	105,000		320,000		210,000		638,000
Interest expense	190,000	[2]	353,000	[2]	390,000	[2]	702,000	[2],[4]
Provision (benefit) for income taxes	(136,000)	[2]	1,033,000	[2]	(258,000)	[2]	1,065,000	[2],[4]
Assets	244,943,000	[4]	404,149,000	[3]	244,943,000	[4]	404,149,000	[3]
Capital investments	6,766,000	[3]	9,069,000	[2]	11,022,000	[3]	22,878,000	[3]
Parent [Member]
Assets	$ 5,345,762,000				$ 5,345,762,000				$ 4,809,628,000

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

[3] Capital investments includes increases of $7.5 million and $0.2 million for the three-month periods ended June 30, 2013 and 2012, respectively, and increases of $40.4 million and $15.5 million for the six-month periods ended June 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.