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Fair Value Measurements (Details 2) (USD $)
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9 Months Ended | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | ||||||||||||||
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Sep. 30, 2012
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Sep. 30, 2011
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Dec. 31, 2011
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Sep. 30, 2012
Net Profits Plan liability
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Sep. 30, 2011
Net Profits Plan liability
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Mar. 31, 2011
Net Profits Plan liability
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Sep. 30, 2012
Net Profits Plan liability
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Sep. 30, 2012
Fair Value, Measurements, Recurring [Member]
Fair Value, Inputs, Level 3 [Member]
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Dec. 31, 2011
Fair Value, Measurements, Recurring [Member]
Fair Value, Inputs, Level 3 [Member]
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Sep. 30, 2012
Net Profits Plan [Member]
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Sep. 30, 2011
Net Profits Plan [Member]
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Sep. 30, 2012
Net Profits Plan [Member]
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Sep. 30, 2011
Net Profits Plan [Member]
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| Liability measured at fair value using Level 3 inputs | |||||||||||||||||||
| Net Profits Plan Liability | $ 90,389,000 | [1] | $ 107,731,000 | [1] | |||||||||||||||
| Net Profits Plan | |||||||||||||||||||
| Discount rate to calculate liability pools currently in payout (as a percent) | 12.00% | 12.00% | |||||||||||||||||
| Discount rate to calculate liability pools not currently in payout (as a percent) | 15.00% | 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 | 8,000,000 | 8,000,000 | |||||||||||||||||
| Percentage increase in discount rate (as a percent) | 1.00% | ||||||||||||||||||
| Decrease in liability due to a 1 percent increase in discount rate | 4,000,000 | 4,000,000 | |||||||||||||||||
| Liability measured at fair value using Level 3 inputs | |||||||||||||||||||
| Beginning balance | 107,731,000 | 135,850,000 | 133,419,000 | 89,591,000 | |||||||||||||||
| Net increase (decrease) in liability | (1,753,000) | (2,001,000) | 5,558,000 | (20,194,000) | |||||||||||||||
| Net settlements | (15,589,000) | (25,360,000) | (4,760,000) | (4,736,000) | |||||||||||||||
| Transfers in (out) of Level 3 | 0 | 0 | 0 | 0 | |||||||||||||||
| Ending balance | 90,389,000 | 108,489,000 | 90,389,000 | 108,489,000 | 90,389,000 | ||||||||||||||
| Amount of cash payments related to divested properties | 274,000 | 0 | 2,000,000 | 6,300,000 | |||||||||||||||
| Amount paid for settlement of liabilities | 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 | 5,197,761,000 | 4,378,987,000 | 2,057,000 | [2] | 139,992,000 | [2] | |||||||||||||
| Unproved Oil and Gas Properties [Abstract] | |||||||||||||||||||
| Unproved Oil and Gas Property, Successful Effort Method | $ 160,468,000 | $ 120,966,000 | $ 9,200,000 | [2] | $ 15,809,000 | [2] | |||||||||||||
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