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Bank borrowings (Details) - USD ($)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Debt Instrument [Line Items]      
Bank borrowings $ 19,924,000 $ 11,324,000  
Net interest expense capitalized $ 890,000 $ 470,000 $ 0
Minimum | Loan Prime Rate      
Debt Instrument [Line Items]      
Spread on variable basis rate 15.00%    
Maximum | Loan Prime Rate      
Debt Instrument [Line Items]      
Spread on variable basis rate 30.00%