Quarterly report pursuant to Section 13 or 15(d)

Subsequent Event (Preliminary Purchase Price Allocation) (Parenthetical) (Detail)

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Subsequent Event (Preliminary Purchase Price Allocation) (Parenthetical) (Detail) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2012
Dec. 31, 2011
Subsequent Event [Line Items]    
Allocation to unproved properties $ 158,585 $ 154,516
Oil And Gas [Member]
   
Subsequent Event [Line Items]    
Allocation to unproved properties $ 13,100