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Summary of significant accounting policies, Revenue recognition - Contract balances (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Summary of significant accounting policies [Abstract]    
Revenue recognized $ 1,239  
Contract liabilities [Abstract]    
Customer pre-payments 50 $ 79
Deferred revenue, current 384 169
Deferred revenue, non-current 265 69
Total contract liabilities $ 699 $ 317