Quarterly report pursuant to Section 13 or 15(d)

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

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Derivative Financial Instruments (Changes in Fair Value of Derivative Contracts Recognized in Earnings) (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Derivatives, Fair Value [Line Items]        
Realized (gain) loss $ 875 $ 917 $ 6,960 $ 9,239
Unrealized (gain) loss 23,784 (18,240) 7,461 (20,054)
Total $ 24,659 $ (17,323) $ 14,421 $ (10,815)