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Fair Value Measurements - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Unrealized losses (gains) on derivatives $ 22,417 $ 127,836
Fair Value Measurements Level 3
   
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Unrealized losses (gains) on derivatives $ 900 $ (1,600)