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Summary of significant accounting policies - Variable interest entity (Details) - Beijing WFOE
12 Months Ended
Dec. 31, 2019
Shareholders' Voting Rights Proxy Agreement  
Significant accounting policies  
Power of attorney, term 10 years
Notice period for termination of agreement 30 days
Power of attorney, renewal period 1 year
Exclusive Business Cooperation Agreements  
Significant accounting policies  
Extended term of agreement 10 years
Percentage of consolidated profit, base for fees on technical support and consulting services 100.00%