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Summary of Significant Accounting Policies - Intangible Assets Subject to Amortization (Details) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Accounting Policies [Abstract]        
Impairment of intangible assets (excluding goodwill) $ 0 $ 0 $ 0 $ 0