Segment Information (Details) (USD $)	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013		Dec. 31, 2013
Segment Reporting Information [Line Items]
Revenues	$ 1,112,777,000		$ 733,649,000
Operating income (loss)	434,619,000		251,960,000	[1]
Other loss, net	1,193,000		(533,000)
Loss on derivatives	(99,720,000)		(29,794,000)
Depreciation, depletion and amortization	225,076,000		179,467,000
Interest expense	12,907,000	[1]	9,021,000	[1]
Provision (benefit) for income taxes	128,995,000	[1]	85,097,000	[1]
Assets	8,423,164,000		6,916,791,000		8,047,726,000
Capital investments	541,936,000	[2]	518,056,000	[2]
Change in accrued expenditures	5,600,000		32,900,000
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	918,527,000		498,935,000
Exploration and Production [Member]
Segment Reporting Information [Line Items]
Revenues	795,413,000		510,094,000
Operating income (loss)	352,140,000		175,758,000	[1]
Other loss, net	1,512,000		(378,000)
Loss on derivatives	(99,242,000)		(29,794,000)
Depreciation, depletion and amortization	211,080,000		167,450,000
Interest expense	9,116,000	[1]	6,176,000	[1]
Provision (benefit) for income taxes	97,767,000	[1]	55,816,000	[1]
Assets	6,621,342,000		5,389,728,000
Capital investments	498,907,000	[2]	475,333,000	[2]
Exploration and Production [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	6,405,000		1,508,000
Exploration and Production [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Assets	79,900,000		45,900,000
Capital investments	4,100,000		2,300,000
Midstream Services [Member]
Segment Reporting Information [Line Items]
Revenues	317,326,000		223,534,000
Operating income (loss)	82,639,000		76,307,000	[1]
Other loss, net	47,000		(155,000)
Loss on derivatives	(362,000)
Depreciation, depletion and amortization	13,953,000		11,912,000
Interest expense	3,597,000	[1]	2,645,000	[1]
Provision (benefit) for income taxes	31,507,000	[1]	29,403,000	[1]
Assets	1,543,944,000		1,278,256,000
Capital investments	38,553,000	[2]	38,467,000	[2]
Midstream Services [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	912,081,000		497,373,000
Other [Member]
Segment Reporting Information [Line Items]
Revenues	38,000		21,000
Operating income (loss)	(160,000)		(105,000)	[1]
Other loss, net	(366,000)
Loss on derivatives	(116,000)
Depreciation, depletion and amortization	43,000		105,000
Interest expense	194,000	[1]	200,000	[1]
Provision (benefit) for income taxes	(279,000)	[1]	(122,000)	[1]
Assets	257,878,000	[3]	248,807,000	[3]
Capital investments	4,476,000	[2]	4,256,000	[2]
Other [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	41,000		54,000
Marketing [Member] | Midstream Services [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues	$ 823,500,000		$ 419,500,000

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

[2] Capital investments includes increases of $5.6 million and $32.9 million for the three-month periods ended March 31, 2014 and 2013, respectively, relating to the change in accrued expenditures between periods.

[3] Other assets represent corporate assets not allocated to segments and assets, including restricted cash and investments in cash equivalents, for non reportable segments.