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Assumptions Used to Estimate Fair Value of Employee Stock Purchase Plan (Detail) - Employee stock purchase plan
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected dividend yield 0.00% 0.00%
Expected volatility, Minimum 35.00% 36.00%
Expected volatility, Maximum 46.00% 57.00%
Risk-free interest rate, Minimum 0.50% 0.30%
Risk-free interest rate, Maximum 0.70% 0.50%
Expected forfeiture 0.00% 0.00%
Contractual term of up to 24 months 24 months