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Summary of significant accounting policies - Share-based compensation (Details) - USD ($)
$ / shares in Units, $ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2020
Dec. 31, 2019
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
Service-based options      
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]      
Award expiration period 10 years 10 years  
Expected life of options   6 years 3 months 6 years 3 months
Expected volatility, Minimum   45.10% 44.80%
Expected volatility, Maximum   45.30% 46.60%
Risk-free interest rates, Minimum   1.60% 2.50%
Risk-free interest rates, Maximum   1.70% 2.80%
Weighted-average grant-date fair value   $ 7.29 $ 2.69
Performance-based options      
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]      
Award expiration period 10 years 10 years  
Expected life of options     4 years 6 months
Expected volatility     55.00%
Expected volatility, Minimum   50.00%  
Expected volatility, Maximum   55.00%  
Risk-free interest rates     2.70%
Risk-free interest rates, Minimum   1.49%  
Risk-free interest rates, Maximum   1.67%  
Expected dividend yield     0.00%
Weighted-average grant-date fair value   $ 6.02 $ 1.91
Threshold cash return on investment upon termination event to determine vesting of performance shares $ 1,515 $ 1,515  
Minimum | Performance-based options      
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]      
Expected life of options   3 years  
Maximum | Performance-based options      
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]      
Expected life of options   3 years 3 months