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Fair Value Measurements (Details 2) (USD $)
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6 Months Ended | 3 Months Ended | 6 Months Ended | 3 Months Ended | 6 Months Ended | ||||||||||||||
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Jun. 30, 2012
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Jun. 30, 2011
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Dec. 31, 2011
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Jun. 30, 2012
Net Profits Plan liability
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Jun. 30, 2011
Net Profits Plan liability
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Jun. 30, 2012
Net Profits Plan liability
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Dec. 31, 2010
Net Profits Plan liability
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Jun. 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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Jun. 30, 2012
Net Profits Plan [Member]
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Jun. 30, 2011
Net Profits Plan [Member]
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Jun. 30, 2012
Net Profits Plan [Member]
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Jun. 30, 2011
Net Profits Plan [Member]
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| Liability measured at fair value using Level 3 inputs | |||||||||||||||||||
| Net Profits Plan Liability | $ 89,591,000 | [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 | 111,670,000 | 147,403,000 | |||||||||||||||
| Net increase (decrease) in liability | (7,311,000) | 18,193,000 | (16,473,000) | (6,092,000) | |||||||||||||||
| Net settlements | (10,829,000) | (20,624,000) | (5,606,000) | (7,892,000) | |||||||||||||||
| Transfers in (out) of Level 3 | 0 | 0 | 0 | 0 | |||||||||||||||
| Ending balance | 89,591,000 | 133,419,000 | 89,591,000 | 133,419,000 | 89,591,000 | 147,403,000 | |||||||||||||
| Amount of cash payments related to divested properties | 1,400,000 | 2,000,000 | 1,700,000 | 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 | 4,869,603,000 | 4,378,987,000 | 79,543,000 | [2] | 139,992,000 | [2] | |||||||||||||
| Unproved Oil and Gas Properties [Abstract] | |||||||||||||||||||
| Unproved Oil and Gas Property, Successful Effort Method | $ 122,005,000 | $ 120,966,000 | $ 15,650,000 | [2] | $ 15,809,000 | [2] | |||||||||||||
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