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Commitments and Contingencies - Effect of Financial Derivatives on Statements of Income (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Foreign Exchange Contracts [Member] | Derivatives not Designated as Hedging Instruments [Member] | Other Expense (Income) [Member]      
Derivative Instruments, Gain (Loss) [Line Items]      
Amount of Gain or (Loss) Recognized in Income on Derivatives $ (1,847) $ 1,566 $ 366