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Summary of Significant Accounting Policies, Advertising (FY) (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Accounting Policies [Abstract]              
Advertising expenses $ 4.1 $ 2.4 $ 11.9 $ 6.7 $ 10.6 $ 20.6 $ 6.6