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Share-based compensation - Weighted average assumptions (Details) - SARs and TSARs
12 Months Ended
Dec. 31, 2018
USD ($)
year
Dec. 31, 2017
USD ($)
year
Disclosure of terms and conditions of share-based payment arrangement [line items]    
Risk-free interest rate 2.60% 1.80%
Expected dividend yield 2.70% 2.00%
Expected life of SARs and TSARs (years) | year 1.5 1.2
Expected volatility 35.00% 31.00%
Expected forfeitures 0.20% 0.20%
Weighted average fair value (USD per share) | $ $ 7.93 $ 19.02