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Summary of Significant Accounting Policies - Accounts Receivable Allowances (Narrative) (Details) - USD ($)
Dec. 31, 2020
Dec. 31, 2019
Summary of Significant Accounting Policies    
Accounts Receivable, Allowance for Credit Loss $ 41,000 $ 50,000
Accounts Receivable, Allowance for Credit Loss, Chargebacks $ 179,000 $ 476,000