Segment Information (Details) (USD $)	3 Months Ended										12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011		Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Revenues from external customers	$ 772,998,000		$ 685,848,000	$ 599,728,000	$ 656,469,000	$ 744,150,000		$ 767,255,000	$ 765,166,000	$ 676,335,000	$ 2,715,043,000		$ 2,952,906,000		$ 2,610,663,000
Intersegment revenues											1,599,524,000		2,010,369,000		1,734,605,000
Operating income (loss)	(577,304,000)		(71,838,000)	(652,394,000)	185,960,000	263,890,000		295,718,000	282,542,000	232,651,000	(1,115,576,000)	[1]	1,074,801,000		1,021,228,000
Other Income (Loss), Net											1,030,000		264,000		427,000
Depreciation, depletion and amortization expense											810,953,000		704,511,000		590,332,000
Impairment of natural gas and oil properties											1,939,734,000
Interest expense											35,657,000	[2]	24,075,000	[2]	26,163,000	[2]
Provision (benefit) for income taxes											(443,139,000)	[2]	413,221,000	[2]	391,659,000	[2]
Assets	6,737,527,000					7,902,897,000					6,737,527,000		7,902,897,000		6,017,463,000
Capital investments											2,080,519,000	[3]	2,207,174,000	[3]	2,120,065,000	[3]
Change in accrued expenditures											(36,900,000)		(4,300,000)		(14,400,000)
Intersegment revenues for marketing company's E&P sales											1,300,000,000		1,700,000,000		1,500,000,000
Capital investment related to companys E&P segment related to Canada for December 31, 2012 and 2011											11,600,000		18,700,000
Assets related to E&P program in Canada	44,400,000					28,400,000					44,400,000		28,400,000
Non-cash ceiling test impairment											1,939,700,000
Exploration and Production [Member]
Revenues from external customers											1,949,674,000		2,088,763,000		1,871,835,000
Intersegment revenues											(1,452,000)		11,725,000		18,609,000
Operating income (loss)											(1,411,211,000)	[1]	825,138,000		829,462,000
Other Income (Loss), Net											(1,156,000)		328,000		235,000
Depreciation, depletion and amortization expense											765,368,000		666,125,000		561,018,000
Impairment of natural gas and oil properties											1,939,734,000
Interest expense											20,315,000	[2]	9,026,000	[2]	7,888,000	[2]
Provision (benefit) for income taxes											(548,556,000)	[2]	322,714,000	[2]	323,748,000	[2]
Assets	5,193,733,000	[4]				6,547,117,000	[4]				5,193,733,000	[4]	6,547,117,000	[4]	4,849,478,000	[4]
Capital investments											1,860,681,000	[3]	1,977,493,000	[3]	1,775,518,000	[3]
Midstream Services [Member]
Revenues from external customers											765,255,000		864,096,000		738,828,000
Intersegment revenues											1,598,225,000		1,995,423,000		1,715,012,000
Operating income (loss)											294,302,000	[1]	247,952,000		191,566,000
Other Income (Loss), Net											132,000		(91,000)		179,000
Depreciation, depletion and amortization expense											44,395,000		37,261,000		28,765,000
Interest expense											14,341,000	[2]	15,049,000	[2]	18,275,000	[2]
Provision (benefit) for income taxes											104,522,000	[2]	90,221,000	[2]	67,834,000	[2]
Assets	1,273,228,000					1,119,861,000					1,273,228,000		1,119,861,000		1,016,563,000
Capital investments											164,978,000	[3]	160,776,000	[3]	271,316,000	[3]
Other [Member]
Revenues from external customers											114,000		47,000
Intersegment revenues											2,751,000		3,221,000		984,000
Operating income (loss)											1,333,000	[1]	1,711,000		200,000
Other Income (Loss), Net											2,054,000		27,000		13,000
Depreciation, depletion and amortization expense											1,190,000		1,125,000		549,000
Interest expense											1,001,000	[2]
Provision (benefit) for income taxes											895,000	[2]	286,000	[2]	77,000	[2]
Assets	270,566,000					235,919,000					270,566,000		235,919,000		151,422,000
Capital investments											$ 54,860,000	[3]	$ 68,905,000	[3]	$ 73,231,000	[3]

[1] The operating loss for the E&P segment for the twelve months ended December 31, 2012 includes a $1,939.7 million non-cash ceiling test impairment of our natural gas and oil properties.

[2] Interest income, interest expense and the provision (benefit) for income taxes by segment are an allocation of corporate amounts as cash equivalents, debt and income tax expense allocated as they are incurred at the corporate level.

[3] Capital investments include a decrease of $36.9 million for 2012, an increase of $4.3 million for 2011 and an increase of $14.4 million for 2010 related to the change in accrued expenditures between years.

[4] Includes capital investments for office, technology, drilling rigs and other ancillary equipment not directly related to natural gas and oil property acquisition, exploration and development activities.