Supplemental Oil and Gas Disclosures (Unaudited), Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013 people		Dec. 31, 2012		Dec. 31, 2011
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Period for standardize measure of discounted future net cash flows	12 months
Number of years of experience of Evaluator	35 years
Number of engineers	4
Discount rate (in hundredths)	10.00%
Future net cash inflows after income taxes discounted at 10% annual discount rate to arrive at Standardized Measure [Abstract]
Future cash inflows	$ 2,244,846		$ 1,784,920		$ 1,471,352
Future production costs	(754,722)		(642,706)		(544,970)
Future development costs	(467,206)		(328,554)		(228,804)
Future income tax expense	(244,394)		(149,625)		(106,839)
Future net cash flows	778,524		664,035		590,739
Discount	(437,539)		(385,890)		(321,657)
Standardized Measure of discounted future net cash relating to proved reserves	340,985		278,145		269,082
Analysis of the changes in Standardized Measure [Roll Forward]
Standardized Measure, beginning of year	278,145		269,082		196,993
Sales and transfers of oil and gas produced, net of production costs	60,403		37,080		37,171
Net change in prices and development and production costs from prior year	169,969		60,710		92,886
Extensions, discoveries, and improved recovery, less related costs	156,456		73,236		47,765
Sales of minerals in place	(125,533)		(20,089)		0
Purchased of minerals in place	0		131		0
Revisions of previous quantity estimates	2,930		3,355		1,329
Change in timing and other	(62,861)		(88,309)		(23,501)
Change in future income tax expense	(45,532)		(9,799)		(28,918)
Accretion of discount	27,814		26,908		19,699
Standardized Measure, end of year	340,985		278,145		269,082
Oil and gas prices considered in standardized measure of discounted future net cash flows [Abstract]
Oil (in dollars per Bbl)	97.33	[1]	95.14	[1]	96.19	[1]
Gas (in dollars per MMbtu)	3.67	[2]	2.86	[2]	4.16	[2]
Oil (in dollars per Bbl)	95.90	[3]	88.26	[3]	88.58	[3]
Gas (in dollars per MMBtu)	3.65	[4]	2.61	[4]	3.73	[4]
NGL's (in dollars per Bbl)	31.98	[5]	36.76	[5]	50.21	[5]
U.S. [Member]
Future net cash inflows after income taxes discounted at 10% annual discount rate to arrive at Standardized Measure [Abstract]
Future cash inflows	2,234,632		1,766,515		1,420,013
Future production costs	(751,058)		(634,903)		(532,056)
Future development costs	(463,456)		(324,704)		(224,254)
Future income tax expense	(244,394)		(149,625)		(104,279)
Future net cash flows	775,724		657,283		559,424
Discount	(436,077)		(383,271)		(310,516)
Standardized Measure of discounted future net cash relating to proved reserves	339,647		274,012		248,908
Canada [Member]
Future net cash inflows after income taxes discounted at 10% annual discount rate to arrive at Standardized Measure [Abstract]
Future cash inflows	10,214		18,405		51,339
Future production costs	(3,664)		(7,803)		(12,914)
Future development costs	(3,750)		(3,850)		(4,550)
Future income tax expense	0		0		(2,560)
Future net cash flows	2,800		6,752		31,315
Discount	(1,462)		(2,619)		(11,141)
Standardized Measure of discounted future net cash relating to proved reserves	$ 1,338		$ 4,133		$ 20,174

[1] The quoted oil price for the year ended December 31, 2011, 2012 and 2013 is the 12-month average first-day-of-the-month West Texas Intermediate spot price for each month of 2011, 2012 and 2013.

[2] The quoted gas price for the year ended December 31, 2011, 2012 and 2013 is the 12-month average first-day-of-the-month Henry Hub spot price for each month of 2011, 2012 and 2013.

[3] The oil price is the realized price at the wellhead as of December 31 of each year after the appropriate differentials have been applied.

[4] The gas price is the realized price at the wellhead as of December 31 of each year after the appropriate differentials have been applied.

[5] The NGL price is the realized price as of December 31 of each year after the appropriate differentials have been applied.