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Commitments - Reconciliation of Liability for Plugging and Abandonment Costs (Detail) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Commitments And Contingencies Disclosure [Abstract]    
Asset retirement obligation at beginning of period $ 9,012 $ 19,013
Liabilities incurred 338 733
Loss on settlement of obligations 40  
Liabilities settled (776) (15,361)
Accretion expense 394 1,053
Revisions in estimated liabilities 1,529 3,574
Asset retirement obligation at end of period $ 10,537 $ 9,012