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Summary of significant accounting policies - Share-based compensation (Details) - USD ($)
$ in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2020
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]    
Threshold cash return on investment upon termination event to determine vesting of performance shares   $ 1,515.0
Fair value prior to plan modification $ 13.8  
Fair value after plan modification $ 33.0  
Service-based options    
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]    
Award expiration period 10 years  
Performance-based options    
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]    
Award expiration period 10 years  
Threshold cash return on investment upon termination event to determine vesting of performance shares $ 1,515.0