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Fair Value of Financial Instruments - Additional Information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Fair value transfers between level 1 to level 2 $ 0 $ 0
Fair value transfers between level 2 to level 1 0 0
Fair value transfers into level 3 0 0
Fair value transfers out of level 3 0 0
Level II [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Severance pay assets $ 6,600,000 $ 5,100,000