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Fair Value Measurements - Reconciliation of Changes in Fair Value of Level 3 Derivative Instruments (Detail) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Balance $ (5,050) $ (10,486)
Included in earnings   1,595
Purchases, issuances and settlements   3,841
Transfers in and out of Level 3 5,050  
Balance   (5,050)
Change in unrealized gains (losses) included in earnings relating to derivatives still held at December 31   $ 5,436