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Fair Value Measurements - Reconciliation of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis using Significant Unobservable Inputs (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]      
Balance of Level 3, Beginning Balance $ (4,252) $ (222,554) $ 37,854
Total realized and unrealized gains (losses) (5,460) 40,907 (67,412)
Purchases 5,697   23,196
Settlements (received) paid 3,503 60,127 (216,192)
Transfers   117,268 [1]  
Balance of Level 3, Ending Balance (512) (4,252) (222,554)
Commodity derivatives contracts
      
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]      
Balance of Level 3, Beginning Balance (4,252) (205,860) 46,153
Total realized and unrealized gains (losses) (5,460) 44,075 (50,872)
Purchases 5,697   23,196
Settlements (received) paid 3,503 50,713 (224,337)
Transfers   106,820 [1]  
Balance of Level 3, Ending Balance (512) (4,252) (205,860)
Interest rate swaps
      
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]      
Balance of Level 3, Beginning Balance 0 (16,694) (8,299)
Total realized and unrealized gains (losses) 0 (3,168) (16,540)
Purchases 0   0
Settlements (received) paid 0 9,414 8,145
Transfers   10,448 [1]  
Balance of Level 3, Ending Balance $ 0 $ 0 $ (16,694)
[1] Fair values related to the Company’s oil and natural gas fixed price swaps, natural gas collars and interest rate swap were transferred from Level 3 to Level 2 in the fourth quarter of 2011 due to enhancements to the Company’s internal valuation process, including the use of observable inputs to assess the fair value. During the years ended December 31, 2012 and 2010, the Company did not have any transfers between Level 1, Level 2 or Level 3 fair value measurements. The Company’s policy is to recognize transfers between fair value hierarchy levels as of the end of the quarterly reporting period in which the event or change in circumstances causing the transfer occurred.