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Summary of Significant Accounting Policies - Accounts Receivable (Details) - USD ($)
$ in Thousands
Mar. 31, 2020
Dec. 31, 2019
Accounts receivable    
Trade accounts receivable $ 5,780 $ 5,454
Unbilled accounts receivable 653 576
Allowance for product returns and price concessions (113) (231)
Accounts receivable, net $ 6,320 $ 5,799