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Effects of Fuel Options Which Were Not Designated as Hedging Instruments (Detail)

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Effects of Fuel Options Which Were Not Designated as Hedging Instruments (Detail) (Not Designated as Hedging Instrument, Fuel Options, USD $)
In Thousands, unless otherwise specified
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Sep. 30, 2012
Dec. 31, 2012
Dec. 31, 2011
Not Designated as Hedging Instrument | Fuel Options
            
Derivative Instruments, Gain (Loss) [Line Items]            
Gain (loss) recognized in other income (expense) $ 430 $ 1,143 $ 1,011 $ 2,858 $ 3,218 $ 2,422