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Weighted Average Assumptions of Options Granted (Detail)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Stock Options      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected volatility 22.20% 22.23% 25.14%
Risk-free interest rate 1.71% 1.07% 1.59%
Dividend yield 0.00% 0.00% 0.00%
Expected term (years) 4 years 9 months 3 days 4 years 7 months 24 days 5 years 7 months 2 days
Employee Stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected volatility 18.21% 23.24% 21.89%
Risk-free interest rate 0.50% 0.16% 0.07%
Dividend yield 0.00% 0.00% 0.00%
Expected term (years) 6 months 6 months 6 months