Registration of securities issued in business combination transactions

FAIR VALUE MEASUREMENTS - Schedule of fair value of warrant liabilities (Q3) (Details)

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FAIR VALUE MEASUREMENTS - Schedule of fair value of warrant liabilities (Q3) (Details) - USD ($)
3 Months Ended 12 Months Ended
Sep. 30, 2021
Jun. 30, 2021
Mar. 31, 2021
Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fair value at beginning     $ 13,365,500 $ 0
Change in fair value       (236,500)
Fair value at ending       13,365,500
New Warrants        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fair value at beginning $ 11,246,750 $ 8,517,250 13,365,500  
Change in fair value (1,070,750) 2,729,500 (4,848,250)  
Fair value at ending 10,176,000 11,246,750 8,517,250 13,365,500
Private Placement Warrants [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fair value at beginning 321,750 237,250 370,500 0
Change in fair value (35,750) 84,500 (133,250) (6,500)
Fair value at ending 286,000 321,750 237,250 370,500
Public Warrants [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fair value at beginning 10,925,000 8,280,000 12,995,000 0
Change in fair value (1,035,000) 2,645,000 (4,715,000) (230,000)
Fair value at ending $ 9,890,000 $ 10,925,000 $ 8,280,000 $ 12,995,000