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Fair Value of Financial Instruments - Additional Information (Detail) - USD ($)
3 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Fair value transfers between level 1 to level 2 $ 0  
Fair value transfers between level 2 to level 1 0  
Fair value transfers into level 3 0  
Fair value transfers out of level 3 0  
Level II [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Severance pay assets $ 6,400,000 $ 6,600,000