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Asset Retirement Obligations (Details) (USD $)
In Thousands, unless otherwise specified
6 Months Ended
Jun. 30, 2014
Asset Retirement Obligation Disclosure [Abstract]  
Future inflation factor used in the calculation of the asset retirement obligations 2.00%
Credit adjusted risk-free interest rate for the calculation of the asset retirement obligations 5.40%
Asset retirement obligations rollforward  
Asset retirement obligations at December 31, 2013 $ 289,321
Liabilities added from drilling 5,673
Current year accretion expense 9,570
Settlements (9,947)
Revision of estimates 9,130
Asset retirement obligations at June 30, 2014 $ 303,747