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Summary of significant accounting policies - Intangible assets (Details)
12 Months Ended
Dec. 31, 2021
Land use rights  
Finite-Lived Intangible Assets [Line Items]  
Estimated useful lives 30 years
Acquired computer software  
Finite-Lived Intangible Assets [Line Items]  
Estimated useful lives 5 years
Audio-visual license  
Finite-Lived Intangible Assets [Line Items]  
Estimated useful lives 9 years