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Share-based compensation (Weighted Average Assumptions for Option Grants) (Details) - Options
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Dividend yield 2.30% 2.70% 2.40%
Expected volatility 21.00% 25.20% 27.80%
Risk-free interest rate 1.70% 1.70% 1.00%
Expected life 5 years 9 months 18 days 5 years 9 months 18 days 5 years 9 months 18 days