Fair Value Measurements (Details 2) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Proved Oil and Gas Properties
Discount rate used for fair value of oil and gas properties (as a percent)		12.00%
Period of New York Mercantile Exchange ("NYMEX") strip pricing used for price forecast (in years)		5
Materials inventory measured at fair value		$ 0
Asset retirement obligations measured at fair value		0
Net Profit Plan liability [Member]
Liability measured at fair value using Level 3 inputs
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%
Number of periods used for price assumptions of strip prices of liabilities		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		9,100,000
Percentage increase in discount rate (as a percent)		1.00%
Increase in liability due to a 1 percent decrease in discount rate		5,100,000
Liability measured at fair value using Level 3 inputs
Net Profits Plan Liability: Beginning Balance	135,850,000	135,850,000	170,291,000	177,366,000
Net increase (decrease) in liability		2,269,000	14,063,000	13,511,000
Net settlements		(30,388,000)	(48,504,000)	(20,586,000)
Transfers in (out) of Level 3		0	0	0
Net Profits Plan Liability: Ending balance		107,731,000	135,850,000	170,291,000
Amount paid for settlement of liabilities	2,600,000
Recurring basis | Level 3
Liability measured at fair value using Level 3 inputs
Deferred Compensation Cash-based Arrangements, Liability, Current and Noncurrent			0
Fair Value, Measurements, Nonrecurring [Member] | Level 3
Proved Oil and Gas Properties
Proved oil and gas Properties measured at fair value		$ 139,992,000