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Stock-based Compensation - Class P Units Activity (Q3) (Details) - shares
3 Months Ended 6 Months Ended 9 Months Ended 12 Months Ended
Jun. 16, 2021
Sep. 30, 2021
Jun. 15, 2021
Sep. 30, 2021
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Number of Units              
Recapitalization in connection with the Business Combination (in shares) 25,687,126            
Class A-3 and Class B Units              
Number of Units              
Outstanding beginning balance (in shares) 262,249   274,816 274,816 264,077 245,371 252,241
Repurchases (in shares)     (8,279)   (1,900)   (12,391)
Cancellations (in shares)     (4,288)   (1,611) (7,284) (38,604)
Outstanding ending balance (in shares)     262,249   274,816 264,077 245,371
Vested (in shares)         234,114 207,398  
Class A-3 Units              
Number of Units              
Conversion to Class P Units in connection with the Business Combination (in shares) (53,333)            
Class P Units              
Number of Units              
Cancellations (in shares)   (216,920)          
Outstanding ending balance (in shares) 25,896,042 25,679,122   25,679,122      
Vested (in shares)       23,197,454