Segment Information (Details) (USD $)	3 Months Ended										12 Months Ended
	Dec. 31, 2013		Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012		Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Revenues from external customers	$ 907,089,000		$ 868,366,000	$ 862,041,000	$ 733,649,000	$ 777,355,000		$ 691,727,000	$ 606,076,000	$ 654,835,000	$ 3,371,145,000		$ 2,729,993,000		$ 2,951,332,000
Intersegment revenues											2,380,043,000		1,599,524,000		2,010,369,000
Operating income (loss)	317,117,000		309,229,000	325,317,000	251,960,000	(572,947,000)		(65,959,000)	(646,046,000)	184,326,000	1,203,623,000		(1,100,626,000)	[1]	1,073,227,000
Other income (loss), net											2,207,000		1,030,000		264,000
Gain (loss) on derivatives											26,141,000		(14,950,000)		1,574,000
Depreciation, depletion and amortization expense											786,612,000		810,953,000		704,511,000
Impairment of natural gas and oil properties													1,939,734,000
Interest expense											41,594,000	[2]	35,657,000	[2]	24,075,000	[2]
Provision (benefit) for income taxes											486,874,000	[2]	(443,139,000)	[2]	413,221,000	[2]
Assets	8,047,726,000					6,737,527,000					8,047,726,000		6,737,527,000		7,902,897,000
Capital investments											2,234,797,000	[3]	2,080,519,000	[3]	2,207,174,000	[3]
Change in accrued expenditures											(24,900,000)		(36,900,000)		(4,300,000)
Intersegment revenues for marketing company's E&P sales											2,000,000,000		1,300,000,000		1,700,000,000
Capital investment related to companys E&P segment related to Canada for December 31, 2012 and 2011											35,400,000		11,600,000		18,700,000
Assets related to E&P program in Canada	79,200,000					44,400,000					79,200,000		44,400,000		28,400,000
Non-cash ceiling test impairment											1,939,700,000
Exploration and Production [Member]
Revenues from external customers											2,398,214,000		1,964,624,000		2,087,189,000
Intersegment revenues											5,951,000		(1,452,000)		11,725,000
Operating income (loss)											878,701,000		(1,396,261,000)	[1]	823,564,000
Other income (loss), net											2,741,000		(1,156,000)		328,000
Gain (loss) on derivatives											25,670,000		(14,950,000)		1,574,000
Depreciation, depletion and amortization expense											735,215,000		765,368,000		666,125,000
Impairment of natural gas and oil properties													1,939,734,000
Interest expense											30,244,000	[2]	20,315,000	[2]	9,026,000	[2]
Provision (benefit) for income taxes											368,320,000	[2]	(548,556,000)	[2]	322,714,000	[2]
Assets	6,356,799,000	[4]				5,193,733,000	[4]				6,356,799,000	[4]	5,193,733,000	[4]	6,547,117,000	[4]
Capital investments											2,052,148,000	[3]	1,860,681,000	[3]	1,977,493,000	[3]
Midstream Services [Member]
Revenues from external customers											972,832,000		765,255,000		864,096,000
Intersegment revenues											2,373,851,000		1,598,225,000		1,995,423,000
Operating income (loss)											325,435,000		294,302,000	[1]	247,952,000
Other income (loss), net											(1,000)		132,000		(91,000)
Gain (loss) on derivatives											480,000
Depreciation, depletion and amortization expense											50,940,000		44,395,000		37,261,000
Interest expense											10,619,000	[2]	14,341,000	[2]	15,049,000	[2]
Provision (benefit) for income taxes											119,223,000	[2]	104,522,000	[2]	90,221,000	[2]
Assets	1,427,294,000					1,273,228,000					1,427,294,000		1,273,228,000		1,119,861,000
Capital investments											157,635,000	[3]	164,978,000	[3]	160,776,000	[3]
Other [Member]
Revenues from external customers											99,000		114,000		47,000
Intersegment revenues											241,000		2,751,000		3,221,000
Operating income (loss)											(513,000)		1,333,000	[1]	1,711,000
Other income (loss), net											(533,000)		2,054,000		27,000
Gain (loss) on derivatives											(9,000)
Depreciation, depletion and amortization expense											457,000		1,190,000		1,125,000
Interest expense											731,000	[2]	1,001,000	[2]
Provision (benefit) for income taxes											(669,000)	[2]	895,000	[2]	286,000	[2]
Assets	263,633,000					270,566,000					263,633,000		270,566,000		235,919,000
Capital investments											$ 25,014,000	[3]	$ 54,860,000	[3]	$ 68,905,000	[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 expense and the provision (benefit) for income taxes by segment are an allocation of corporate amounts as they are incurred at the corporate level.

[3] Capital investments include a decrease of $24.9 million for 2013, a decrease of $36.9 million for 2012 and an increase of $4.3 million for 2011 related to the change in accrued expenditures between years.

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