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Summary of Significant Accounting Policies - Revenue by Geographic Areas (Q3) (Details) - USD ($)
 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
Disaggregation of Revenue [Line Items]              
Total revenues $ 50,884,000 $ 46,505,000 $ 138,969,000 $ 117,470,000 $ 161,790,762 $ 144,231,479 $ 101,402,007
U.S. revenues              
Disaggregation of Revenue [Line Items]              
Total revenues 50,884,000 34,877,000 138,969,000 91,275,000 130,373,022 132,076,823 100,670,847
Foreign revenues              
Disaggregation of Revenue [Line Items]              
Total revenues $ 0 $ 11,628,000 $ 0 $ 26,195,000 $ 31,417,740 $ 12,154,656 $ 731,160