Acquisition of Working Interests in Natural Gas Properties - Schedule of Estimated Fair Values of Assets Acquired and Liabilities Assumed at Acquisition Date (Detail) (USD $) In Thousands, unless otherwise specified	Oct. 31, 2012
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Accounts receivable	$ 1,107
Property, plant and equipment	49,317
Total assets acquired	50,424
Accounts payable	126
Noncurrent accrued and other liabilities	79
Total liabilities assumed	205
Net assets acquired	$ 50,219