Annual report pursuant to Section 13 and 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
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Derivative (gain) loss:      
Realized $ 8,589 $ 7,665 $ 9,873
Unrealized (119) 6,289 (11,769)
Total $ 8,470 $ 13,954 $ (1,896)