Segment Information (Details) (USD $)	3 Months Ended
	Mar. 31, 2012		Mar. 31, 2011		Dec. 31, 2011
Revenues from external customers	$ 656,469,000		$ 676,335,000
Intersegment revenues	357,813,000		479,879,000
Operating income	185,960,000		232,651,000
Other Income (Loss), Net	(200,000)	[1]	374,000	[1]
Depreciation, depletion and amortization expense	193,627,000		163,447,000
Interest expense	7,338,000	[1]	7,436,000	[1]
Provision for income taxes	70,718,000	[1]	88,980,000	[1]
Assets	8,554,161,000	[2]	6,340,748,000		7,902,897,000
Capital investments	573,112,000	[3]	530,529,000	[3]
Change in accrued expenditures	15,300,000		1,500,000
Intersegment revenues for marketing company's E&P sales	286,100,000		411,200,000
Capital investment related to companys E&P segment related to Canada for March 31st 2012 and 2011	2,400,000
Assets related to E&P program in Canada	31,300,000		13,700,000
Exploration and Production [Member]
Revenues from external customers	466,265,000		470,656,000
Intersegment revenues	713,000		5,514,000
Operating income	116,243,000		178,283,000
Other Income (Loss), Net	(183,000)	[1]	343,000	[1]
Depreciation, depletion and amortization expense	182,739,000		154,810,000
Interest expense	3,322,000	[1]	2,904,000	[1]
Provision for income taxes	44,838,000	[1]	69,382,000	[1]
Assets	7,038,247,000		5,134,657,000	[2]
Capital investments	533,139,000	[3]	468,212,000	[3]
Midstream Services [Member]
Revenues from external customers	190,173,000		205,679,000
Intersegment revenues	356,281,000		473,589,000
Operating income	69,289,000		53,917,000
Other Income (Loss), Net	(18,000)	[1]	29,000	[1]
Depreciation, depletion and amortization expense	10,570,000		8,391,000
Interest expense	3,667,000	[1]	4,532,000	[1]
Provision for income taxes	25,848,000	[1]	19,420,000	[1]
Assets	1,087,020,000		1,025,072,000
Capital investments	26,164,000	[3]	45,978,000	[3]
Other [Member]
Revenues from external customers	31,000		0
Intersegment revenues	819,000		776,000
Operating income	428,000		451,000
Other Income (Loss), Net	1,000	[1]	2,000	[1]
Depreciation, depletion and amortization expense	318,000		246,000
Interest expense	349,000	[1]	0	[1]
Provision for income taxes	32,000	[1]	178,000	[1]
Assets	428,894,000	[2]	181,019,000	[2]
Capital investments	$ 13,809,000	[3]	$ 16,339,000	[3]

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

[2] (2) Other assets represent corporate assets not allocated to segments and assets, including investments in cash equivalents, for non-reportable segments.

[3] (3) Capital investments include increases of $15.3 million and $1.5 million for the three months ended March 31, 2012 and 2011, respectively, relating to the change in accrued expenditures between periods.