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Weighted Average Assumptions of Options Granted (Detail)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Stock Options      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected volatility 21.98% 22.20% 22.23%
Risk-free interest rate 2.67% 1.71% 1.07%
Dividend yield 0.00% 0.00% 0.00%
Expected term (years) 5 years 1 month 17 days 4 years 9 months 3 days 4 years 7 months 24 days
Employee Stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected volatility 22.88% 18.21% 23.24%
Risk-free interest rate 1.07% 0.50% 0.16%
Dividend yield 0.00% 0.00% 0.00%
Expected term (years) 6 months 6 months 6 months