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Oil and Gas Producing Activities - Schedule of Information Regarding Costs Incurred for Oil and Gas Property Acquisition, Development and Exploration Activities (Detail) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]      
Unproved properties $ 2,335 $ 17,361 $ 8,931
Proved properties 5,407 5,063 86
Working interest acquisitions   70,827 21,179
Exploration costs 4,550 9,991 2,874
Development costs 285,039 182,522 56,915
Total costs incurred $ 297,331 $ 285,764 $ 89,985