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Summary of Significant Accounting Policies - Additional Information (Q3) (Details)
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2021
USD ($)
Sep. 30, 2020
USD ($)
Sep. 30, 2021
USD ($)
segment
Sep. 30, 2020
USD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Accounting Policies [Line Items]              
Allowance of bad debt $ 3,000,000.0   $ 3,000,000.0   $ 857,382 $ 913,715  
Deferred revenue 7,800,000   7,800,000   5,300,000    
Income tax expense (benefit) (393,000) $ 0 (242,000) $ 0 0 (1,321,045) $ 0
Advertising expense 4,100,000 2,400,000 11,900,000 6,700,000 10,600,000 20,600,000 6,600,000
Political contributions $ 200,000 $ 300,000 $ 500,000 $ 600,000 773,370 $ 915,250 $ 795,275
Number of reportable segments | segment     1        
Tax receivable agreement liabilities as percent of expected benefit 85.00%   85.00%        
Tax receivable agreement, percent recorded in additional paid-in capital 15.00%   15.00%        
Equity securities without readily determinable fair value $ 3,000,000.0   $ 3,000,000.0        
Operating lease, right-of-use asset 37,673,000   37,673,000   43,300,000    
Operating lease liabilities $ 46,069,000   $ 46,069,000   $ 48,400,000    
Class A Common Stock, $0.0001 par value per share              
Accounting Policies [Line Items]              
Payment to continuing members as percent of amount of tax benefit 85.00%   85.00%        
Silver Spike | Legacy WMH Class A Unit holders              
Accounting Policies [Line Items]              
Percentage of voting interests held (over 50%) 50.00%   50.00%