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Accounts receivable, net - Schedule of Accounts Receivable (Details) - USD ($)
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Accounts receivable, net        
Accounts receivable $ 35,137,000 $ 27,100,000    
Less: Allowance for doubtful accounts (7,604,000) (7,709,000) $ (31,000) $ (119,000)
Accounts receivable, net $ 27,533,000 $ 19,391,000