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Commitments and Contingencies - Additional Information (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Commitments And Contingencies Disclosure [Line Items]      
Rent expense $ 5.7 $ 5.0 $ 4.0
Purchase commitments relating to software and services payable in 2018 15.9    
Purchase commitments relating to software and services payable in 2019 16.0    
Operating Expense [Member] | Invest Northern Ireland [Member]      
Commitments And Contingencies Disclosure [Line Items]      
Proceeds from grant 0.6    
Letter of Credit [Member]      
Commitments And Contingencies Disclosure [Line Items]      
Long-term Line of Credit $ 7.7