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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, 2016
Dec. 31, 2015
Dec. 31, 2014
Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions And Weighted Average Fair Values [Line Items]      
Risk-free interest rate 1.30% 1.44% 1.45%
Expected life of option 4 years 10 months 28 days 4 years 10 months 13 days 4 years 10 months 21 days
Expected dividend yield 0.94% 1.12% 1.34%
Expected volatility 21.64% 20.68% 21.44%