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Stock-based compensation (Details)
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Risk free interest rate 0.41% 0.48% 0.47%
Expected option life (years) 1 year 4 months 24 days 1 year 4 months 24 days 1 year 8 months 12 days
Expected volatility 33.74% 48.06% 27.87%
Expected dividend yield 2.35% 2.71% 2.71%