XML 56 R40.htm IDEA: XBRL DOCUMENT v3.8.0.1
Assumptions Used to Estimate Fair Value of Employee Stock Purchase Plan (Detail)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected dividend yield   0.00% [1] 0.00%
Expected volatility, Minimum   38.00% [1] 33.00%
Expected volatility, Maximum   49.00% [1] 49.00%
Risk-free interest rate, Minimum   0.50% [1] 0.20%
Risk-free interest rate, Maximum   2.40% [1] 2.40%
Expected forfeiture     10.00%
Contractual term of up to   10 years [1] 10 years
Employee stock purchase plan      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected dividend yield 0.00% 0.00% 0.00%
Expected volatility, Minimum 28.00% 29.00% 35.00%
Expected volatility, Maximum 46.00% 57.00% 36.00%
Risk-free interest rate, Minimum 0.50% 0.30% 0.10%
Risk-free interest rate, Maximum 1.10% 0.50% 0.30%
Expected forfeiture 0.00% 0.00% 0.00%
Contractual term of up to 24 months 24 months 24 months
[1] During 2016, the Company granted stock options only to its non-employee directors.