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Summary of Significant Accounting Policies - Additional Information (Detail)
 12 Months Ended
Dec. 31, 2017
USD ($)
Customer
shares
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Summary Of Significant Accounting Policy [Line Items]      
Original maturities 90 days    
Number of customers accounted for revenues or accounts receivable greater than 2% or 3% of total | Customer 0    
Annual goodwill impairment test, period November 30th    
Goodwill impairment charges $ 0 $ 0 $ 0
Non-current deferred rent liabilities 7,500,000 7,400,000  
Foreign exchange gains (losses) (600,000) 100,000  
Tax withholding in financing activities $ 2,527,000 2,069,000 1,459,000
Options granted | shares 0    
Unamortized fair value stock based compensation $ 16,600,000    
Weighted average remaining recognition period 2 years 7 months 6 days    
ASU No. 2016-09 [Member]      
Summary Of Significant Accounting Policy [Line Items]      
Cumulative effect adjustment to increase or (reduce) retained earnings   200,000  
Decrease to income tax expense $ 400,000    
Tax withholding in financing activities $ 2,500,000    
Change in reclassification of tax withholding   2,600,000 $ 1,500,000
Early Adoption of ASU No. 2016-16 [Member]      
Summary Of Significant Accounting Policy [Line Items]      
Cumulative effect adjustment to increase or (reduce) retained earnings   $ (500,000)  
Research Service Revenue [Member] | Customer Concentration Risk [Member]      
Summary Of Significant Accounting Policy [Line Items]      
Customer accounted for percentage 2.00%    
Accounts Receivable [Member] | Credit Concentration Risk [Member]      
Summary Of Significant Accounting Policy [Line Items]      
Customer accounted for percentage 3.00%