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Balance Sheet Components - Accounts Receivable (Details) - USD ($)
$ in Thousands
Jun. 30, 2020
Dec. 31, 2019
Jun. 30, 2019
Dec. 31, 2018
Accounts Receivable, after Allowance for Credit Loss [Abstract]        
Accounts receivable, gross $ 5,959 $ 10,938    
Less: allowance for doubtful accounts (34) (23) $ (45) $ (21)
Accounts receivable, net $ 5,925 $ 10,915