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Derivative Instruments and Hedging Activities - Additional Information (Detail) (USD $)
6 Months Ended
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2012
Derivative Instruments And Hedging Activities Disclosure [Abstract]      
Maximum correlation between price of oil & natural gas in market and underlying price basis indicative in the derivative contract 100.00%    
Accumulated other comprehensive income (loss) $ 24,530,000 $ 10,500,000 $ 28,833,000
Accumulated other comprehensive income, to be reclassified into earning in the next twelve months $ 17,200,000