Segment Information (Details) (USD $)	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012		Dec. 31, 2012
Revenues from external customers	$ 733,649,000		$ 654,835,000
Intersegment revenues	498,935,000		357,813,000
Operating income (loss)	251,960,000		184,326,000
Other income (loss), net	(533,000)		(200,000)
Commodity Derivative Income (Loss)	(29,794,000)		1,634,000
Depreciation, depletion and amortization expense	179,467,000		193,627,000
Interest expense	9,021,000	[1]	7,338,000	[1]
Provision (benefit) for income taxes	85,097,000	[1]	70,718,000	[1]
Assets	6,916,791,000		8,554,161,000		6,737,527,000
Capital investments	518,056,000	[2]	573,112,000	[2]
Change in accrued expenditures	32,900,000		15,300,000
Intersegment revenues for marketing company's E&P sales	419,500,000		286,100,000
Capital investment related to companys E&P segment related to Canada for September 30, 2012 and 2011	2,300,000		2,400,000
Assets related to E&P program in Canada	45,900,000		31,300,000
Exploration and Production [Member]
Revenues from external customers	510,094,000		464,631,000
Intersegment revenues	1,508,000		713,000
Operating income (loss)	175,758,000		114,609,000
Other income (loss), net	(378,000)		(183,000)
Commodity Derivative Income (Loss)	(29,794,000)		1,634,000
Depreciation, depletion and amortization expense	167,450,000		182,739,000
Interest expense	6,176,000	[1]	3,322,000	[1]
Provision (benefit) for income taxes	55,816,000	[1]	44,838,000	[1]
Assets	5,389,728,000		7,038,247,000
Capital investments	475,333,000	[2]	533,139,000	[2]
Midstream Services [Member]
Revenues from external customers	223,534,000		190,173,000
Intersegment revenues	497,373,000		356,281,000
Operating income (loss)	76,307,000		69,289,000
Other income (loss), net	(155,000)		(18,000)
Depreciation, depletion and amortization expense	11,912,000		10,570,000
Interest expense	2,645,000	[1]	3,667,000	[1]
Provision (benefit) for income taxes	29,403,000	[1]	25,848,000	[1]
Assets	1,278,256,000		1,087,020,000
Capital investments	38,467,000	[2]	26,164,000	[2]
Other [Member]
Revenues from external customers	21,000		31,000
Intersegment revenues	54,000		819,000
Operating income (loss)	(105,000)		428,000
Other income (loss), net			1,000
Depreciation, depletion and amortization expense	105,000		318,000
Interest expense	200,000	[1]	349,000	[1]
Provision (benefit) for income taxes	(122,000)	[1]	32,000	[1],[3]
Assets	248,807,000	[3]	428,894,000	[3]
Capital investments	$ 4,256,000	[2]	$ 13,809,000	[2]

[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 $32.9 million and $15.3 million for the three-month periods ended March 31, 2013 and 2012, 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.