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Bank borrowings (Details) - USD ($)
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Debt Instrument [Line Items]      
Bank borrowings, current portion $ 2,876,000    
Bank borrowings, non-current portion 17,291,000 $ 19,924,000  
Total 20,167,000 19,924,000  
Net interest expense capitalized $ 1,000,000 $ 890,000 $ 470,000
Minimum | Loan Prime Rate      
Debt Instrument [Line Items]      
Spread on variable basis rate 15.00%