Registration of securities issued in business combination transactions

FAIR VALUE MEASUREMENTS - Key inputs for Private Placement Warrants (Q3) (Details)

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FAIR VALUE MEASUREMENTS - Key inputs for Private Placement Warrants (Q3) (Details) - Private Placement Warrants [Member]
Sep. 30, 2021
$ / shares
d
Dec. 31, 2020
$ / shares
d
Risk-free interest rate    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Warrants measurement input 1.04 0.51
Trading days per year    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Trading days per year | d 250 252
Expected volatility    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Warrants measurement input 14.0 15.8
Exercise price    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Warrants measurement input 11.50 11.50
Stock Price    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Warrants measurement input 9.91 10.08