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Stock compensation plans - Weighted-average assumptions used to estimate fair value of options granted (Detail) - Stock Options
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions And Weighted Average Fair Values [Line Items]      
Risk-free interest rate 1.16% 2.44% 2.67%
Expected life of option 5 years 4 years 11 months 26 days 4 years 11 months 23 days
Expected dividend yield 0.39% 0.47% 0.54%
Expected volatility 23.98% 23.92% 22.65%