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Acquisitions, Acquired Intangible Assets, and Goodwill - Goodwill (Details) - USD ($)
9 Months Ended
Sep. 30, 2021
Dec. 31, 2020
Dec. 31, 2019
Business Combination and Asset Acquisition [Abstract]      
Change in carrying value of goodwill $ 0    
Goodwill $ 1,991,000 $ 1,991,000 $ 0