Changes to Company's Asset Retirement Obligations (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011
Reconciliation of Changes in Asset Retirement Obligations [Line Items]
Balance at beginning of year	$ 3,455	$ 3,063
Liabilities incurred	182	152
Liabilities settled	(83)	(80)
Accretion expense	79	70
Revision in estimated costs of plugging oil and natural gas wells	536	(2)
Asset retirement obligation at end of period	$ 4,169	$ 3,203