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Fair Value Measurements (Details) (USD $)
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Sep. 30, 2012
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Dec. 31, 2011
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| Assets: | ||||||||
| Proved Oil and Gas Property, Successful Effort Method | $ 5,197,761,000 | $ 4,378,987,000 | ||||||
| Unproved Oil and Gas Property, Successful Effort Method | 160,468,000 | 120,966,000 | ||||||
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Convertible Debt [Member]
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| Liabilities [Abstract] | ||||||||
| Interest rate on notes (as a percent) | 3.50% | 3.50% | ||||||
| Approximate estimated fair value of notes | 394,000,000 | |||||||
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6.625% Senior Notes [Member]
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| Liabilities [Abstract] | ||||||||
| Interest rate on notes (as a percent) | 6.625% | 6.625% | ||||||
| Approximate estimated fair value of notes | 371,000,000 | 359,000,000 | ||||||
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6.50% Senior Notes [Member]
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| Liabilities [Abstract] | ||||||||
| Interest rate on notes (as a percent) | 6.50% | 6.50% | ||||||
| Approximate estimated fair value of notes | 370,000,000 | 360,000,000 | ||||||
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6.50% Senior Notes due 2023 [Member]
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| Liabilities [Abstract] | ||||||||
| Interest rate on notes (as a percent) | 6.50% | |||||||
| Approximate estimated fair value of notes | 422,000,000 | |||||||
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Fair Value, Measurements, Recurring [Member] | Level 1
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| Assets: | ||||||||
| Proved Oil and Gas Property, Successful Effort Method | 0 | [1] | ||||||
| Unproved Oil and Gas Property, Successful Effort Method | 0 | [1] | ||||||
| Liabilities [Abstract] | ||||||||
| Net Profits Plan Liability | 0 | [2] | 0 | [2] | ||||
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Fair Value, Measurements, Recurring [Member] | Level 1 | Derivatives not designated as hedging instruments
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| Assets: | ||||||||
| Derivatives | 0 | [2] | 0 | [2] | ||||
| Liabilities [Abstract] | ||||||||
| Derivatives | 0 | [2] | 0 | [2] | ||||
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Fair Value, Measurements, Recurring [Member] | Level 2
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| Assets: | ||||||||
| Proved Oil and Gas Property, Successful Effort Method | 0 | [1] | 0 | [1] | ||||
| Unproved Oil and Gas Property, Successful Effort Method | 0 | [1] | 0 | [1] | ||||
| Liabilities [Abstract] | ||||||||
| Net Profits Plan Liability | 0 | [2] | 0 | [2] | ||||
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Fair Value, Measurements, Recurring [Member] | Level 2 | Derivatives not designated as hedging instruments
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| Assets: | ||||||||
| Derivatives | 64,248,000 | [2] | 86,875,000 | [2] | ||||
| Liabilities [Abstract] | ||||||||
| Derivatives | 28,154,000 | [2] | 55,681,000 | [2] | ||||
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Fair Value, Measurements, Recurring [Member] | Level 3
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| Assets: | ||||||||
| Proved Oil and Gas Property, Successful Effort Method | 2,057,000 | [1] | 139,992,000 | [1] | ||||
| Unproved Oil and Gas Property, Successful Effort Method | 9,200,000 | [1] | 15,809,000 | [1] | ||||
| Liabilities [Abstract] | ||||||||
| Net Profits Plan Liability | 90,389,000 | [2] | 107,731,000 | [2] | ||||
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Fair Value, Measurements, Recurring [Member] | Level 3 | Derivatives not designated as hedging instruments
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| Assets: | ||||||||
| Derivatives | 0 | [2] | 0 | [2] | ||||
| Liabilities [Abstract] | ||||||||
| Derivatives | 0 | [2] | 0 | [2] | ||||
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Fair Value, Measurements, Nonrecurring [Member] | Level 1
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| Assets: | ||||||||
| Proved Oil and Gas Property, Successful Effort Method | 0 | [1] | ||||||
| Unproved Oil and Gas Property, Successful Effort Method | $ 0 | [1] | ||||||
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