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Fair Value Measurements (Details 2) (USD $)
 3 Months Ended
Mar. 31, 2013
Mar. 31, 2013
Net Profits Plan liability
Mar. 31, 2013
Fair Value, Measurements, Recurring [Member]
Fair Value, Inputs, Level 3 [Member]
Dec. 31, 2012
Fair Value, Measurements, Recurring [Member]
Fair Value, Inputs, Level 3 [Member]
Mar. 31, 2013
6.625% Senior Notes Due 2019 [Member]
Dec. 31, 2012
6.625% Senior Notes Due 2019 [Member]
Mar. 31, 2013
6.50% Senior Notes Due 2021 [Member]
Dec. 31, 2012
6.50% Senior Notes Due 2021 [Member]
Mar. 31, 2013
6.50% Senior Notes Due 2023 [Member]
Dec. 31, 2012
6.50% Senior Notes Due 2023 [Member]
Liability measured at fair value using Level 3 inputs                    
Long-term Debt, Fair Value         $ 374,850,000 $ 371,875,000 $ 382,375,000 $ 371,070,000 $ 442,000,000 $ 424,200,000
Net Profits Plan Liability     76,902,000 [1] 78,827,000 [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 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   7,000,000                
Percentage increase in discount rate (as a percent)   1.00%                
Decrease in liability due to a 1 percent increase in discount rate   3,000,000                
Liability measured at fair value using Level 3 inputs                    
Beginning balance   78,827,000                
Net increase (decrease) in liability   2,236,000 [2]                
Net settlements   (4,161,000) [2],[3]                
Transfers in (out) of Level 3   0                
Ending balance   $ 76,902,000                
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 Strip Pricing Used for Price Forecast 5 5                
[1] This represents a financial asset or liability that is measured at fair value on a recurring basis.
[2] Net changes in the Company’s Net Profits Plan liability are shown in the Change in Net Profits Plan liability line item of the accompanying statements of operations
[3] (2) Settlements represent cash payments made or accrued under the Net Profits Plan.