Fair Value Measurements (Details 2) (USD $)	3 Months Ended		3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2012 Net Profits Plan liability	Mar. 31, 2011 Net Profits Plan liability	Mar. 31, 2012 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 3 [Member]	Dec. 31, 2011 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 3 [Member]
Liability measured at fair value using Level 3 inputs
Net Profits Plan Liability					$ 111,670,000	$ 107,731,000	[1]
Net Profits Plan
Discount rate to calculate liability pools currently in payout (as a percent)			12.00%
Discount rate to calculate liability pools not currently in payout (as a percent)			15.00%
Period of New York Mercantile Exchange Strip Pricing Used for Price Forecast	5		5
Period used for price assumptions of strip prices of liabilities (in years)			1
Percentage change in commodity prices (as a percent)			5.00%
Change in liability due to change in commodity prices by 5 percent			9,000,000
Percentage increase in discount rate (as a percent)			1.00%
Decrease in liability due to a 1 percent increase in discount rate			5,000,000
Liability measured at fair value using Level 3 inputs
Beginning balance			107,731,000	135,850,000
Net increase (decrease) in liability			9,162,000	24,285,000
Net settlements			(5,223,000)	(12,732,000)
Transfers in (out) of Level 3			0	0
Ending balance			111,670,000	147,403,000
Amount paid for settlement of liabilities			0	2,600,000
Proved Oil and Gas Properties
Discount rate used for fair value of oil and gas properties (as a percent)	12.00%
Proved Oil and Gas Property, Successful Effort Method, Fair Value	0
Proved Oil and Gas Property, Successful Effort Method	4,657,347,000	4,378,987,000				139,992,000	[2]
Unproved Oil and Gas Properties [Abstract]
Unproved Oil and Gas Property, Successful Efforts Method, Fair Value	0
Unproved Oil and Gas Property, Successful Effort Method	130,688,000	120,966,000				15,809,000	[2]
Inventory, Net [Abstract]
Inventory, Noncurrent, Fair Value Disclosure	0	0
Asset Retirement Obligation [Abstract]
Asset Retirement Obligations, Fair Value	$ 0	$ 0

[1] This represents a financial asset or liability that is measured at fair value on a recurring basis.

[2] This represents a non-financial asset or liability that is measured at fair value on a nonrecurring basis.