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Assumptions Used to Estimate Fair Value of Employee Stock Purchase Plan (Detail) - Employee stock purchase plan
6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected dividend yield 0.00% 0.00%
Expected volatility, Minimum 35.00% 35.00%
Expected volatility, Maximum 42.00% 46.00%
Risk-free interest rate, Minimum 0.70% 0.50%
Risk-free interest rate, Maximum 1.60% 0.70%
Contractual term of up to 24 months 24 months