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Summary of Significant Accounting Policies - Concentration (Details) - customer
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Sales | Customer      
Risk and Uncertainties      
Number of customers exceeding 10% of benchmark 0 0 0
Accounts receivable | Credit      
Risk and Uncertainties      
Number of customers exceeding 10% of benchmark 0 0