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Changes to Company's Asset Retirement Obligations (Detail) (USD $)
In Thousands, unless otherwise specified
9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Reconciliation of Changes in Asset Retirement Obligations [Line Items]    
Balance at beginning of period $ 4,422 $ 3,455
Liabilities incurred 276 288
Liabilities settled (119) (106)
Accretion expense 124 121
Revision in estimated costs of plugging oil and natural gas wells   536
Asset retirement obligation at end of period $ 4,703 $ 4,294