XML 61 R51.htm IDEA: XBRL DOCUMENT v3.10.0.1
Assumptions Used to Estimate Fair Value of Employee Stock Purchase Plan (Detail) - Employee stock purchase plan
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
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 38.00% 28.00% 35.00% 28.00%
Expected volatility, Maximum   43.00% 42.00% 46.00%
Risk-free interest rate, Minimum 2.20% 0.60% 0.70% 0.50%
Risk-free interest rate, Maximum   1.10% 2.20% 1.10%
Contractual term of up to 24 months 24 months 24 months 24 months