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Share-based compensation plans - Estimated Fair Value of all SARs Applying Assumptions (Details) - Stock Appreciation Rights (SARs)
12 Months Ended
Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Expected dividend yield 2.70%
Expected stock price volatility 16.60%
Risk-free interest rate 2.60%
Expected life of SARs 6 years