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Income Taxes - Reconciliation of Statutory Tax Rate to Effective Tax Rate (Detail)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Tax Disclosure [Abstract]      
U.S. federal statutory income tax rate 35.00% 35.00% 35.00%
State and local income taxes, net of federal benefit 2.60% 7.50% 5.40%
Foreign income at other than U.S. rates 2.70% 1.10% (0.30%)
Stock-based compensation (19.00%) 3.80% 0.40%
Meals and entertainment 0.40% 0.30% 0.20%
Change in value of preferred stock warrant liabilities   7.60% 7.00%
Research and development credit (2.80%) (3.10%) (1.00%)
Federal deferred tax asset revaluation 0.90%    
Other permanent items 0.90% 0.40% (0.10%)
Change in valuation allowance (0.50%) 0.70%  
Effective income tax rate 20.20% 53.30% 46.60%