Quarterly report pursuant to Section 13 or 15(d)

Changes in Fair Value of Commodity Derivative Contracts Recognized in Earnings (Detail)

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Changes in Fair Value of Commodity Derivative Contracts Recognized in Earnings (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Derivative (gain) loss:    
Realized $ 4,271 $ 5,800
Unrealized (903) 33,834
Total $ 3,368 $ 39,634