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Stock-Based Compensation - Assumptions in Computation of Fair Value (Detail) - Employee stock purchase plan
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Weighted-average expected volatility 32.94% 22.95% 20.74%
Weighted average expected life (in years) 1 year 4 months 9 days 1 year 2 months 26 days 1 year 5 months 4 days
Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Range of dividend yield 1.94% 2.07% 1.97%
Range of risk-free interest rate 0.10% 1.55% 1.79%
Range of expected volatility 19.28% 19.27% 19.04%
Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Range of dividend yield 2.08% 2.09% 2.00%
Range of risk-free interest rate 1.55% 2.58% 2.68%
Range of expected volatility 51.93% 25.55% 24.33%