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Other Long-Term Assets - Debt Offering Costs (Details) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Dec. 31, 2019
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]      
Deferred offering costs, IPO $ 0 $ 1,839 $ 0
Deferred offering costs, credit facility $ 473 $ 630 $ 0