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Income Tax - Additional Information (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Valuation allowance   $ 326  
Period of cumulative pre-tax income 3 years    
Research and development tax credits $ 774 280  
Cash paid for income taxes 19,163 16,740 $ 12,931
Gross unrecognized tax benefits 3,101 1,007  
Other Liabilities, Non current      
Gross unrecognized tax benefits 3,100 1,000  
UNITED KINGDOM      
Valuation allowance 300 $ 300  
Research and development tax credits 1,300    
International      
Unremitted earnings of subsidiaries, foreign 4,800    
Cash paid for income taxes $ 0