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Asset Retirement Obligations - Additional Information (Details) (USD $)
In Thousands, unless otherwise specified
6 Months Ended
Jun. 30, 2013
Reconciliation Of Changes In Asset Retirement Obligations  
Liability settled or disposed in current period $ (75,060)
Bullwinkle Platform
 
Reconciliation Of Changes In Asset Retirement Obligations  
Liability settled or disposed in current period (22,700)
Permian Properties
 
Reconciliation Of Changes In Asset Retirement Obligations  
Liability settled or disposed in current period $ (15,200)