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Summary of significant accounting policies - Share-based compensation (Details) - USD ($)
$ in Millions
6 Months Ended 9 Months Ended 12 Months Ended
Jun. 30, 2020
Sep. 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 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  
Options granted   0  
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