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Changes to Company's Asset Retirement Obligations (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Reconciliation of Changes in Asset Retirement Obligations [Line Items]    
Balance at beginning of period $ 4,422 $ 3,455
Liabilities incurred 71 106
Liabilities settled (13) (68)
Accretion expense 41 37
Revision in estimated costs of plugging oil and natural gas wells   550
Asset retirement obligation at end of period $ 4,521 $ 4,080