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Long-term investments - Additional Information (Details)
1 Months Ended 12 Months Ended
Oct. 31, 2020
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Net unrealized gains on investments held   $ 794,000 $ 10,907,000  
Impairment of long-term investments   794,000 19,831,000 $ 7,794,000
Shanghai Lexiang [Member]        
Percentage of equity interest disposed 4.82      
Consideration for disposal of equity interest     $ 268,000  
Net unrealized gains on investments held   794,000    
Impairment of long-term investments   $ 794,000    
Chengdu Diting [Member]        
Percent of equity interest owned   12.74% 12.74%  
Guangzhou Yuechuan [Member]        
Percent of equity interest owned   9.30% 9.30%  
Zhuhai Qianyou [Member]        
Percent of equity interest owned   19.00% 19.00%