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Assumptions Used to Estimate Fair Value of Employee Stock Purchase Plan (Detail) - Employee stock purchase plan
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Expected dividend yield 0.00% 0.00% 0.00% 0.00%
Expected volatility, Minimum 28.00% 29.00% 28.00% 29.00%
Expected volatility, Maximum 43.00% 48.00% 46.00% 57.00%
Risk-free interest rate, Minimum 0.60% 0.40% 0.50% 0.30%
Risk-free interest rate, Maximum 1.10% 0.50% 1.10% 0.50%
Expected forfeiture 0.00% 0.00% 0.00% 0.00%
Contractual term of up to 24 months 24 months 24 months 24 months