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Stock compensation plans - weighted-average assumptions used to estimate fair value of non-vested shares granted (Detail) - Stock Options
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions And Weighted Average Fair Values [Line Items]      
Risk-free interest rate 1.47% 0.94% 0.94%
Expected dividend yield 0.71% 0.93% 1.12%