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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Forward Purchase Agreement Rollforward (Q1) (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Dec. 31, 2023
Dec. 31, 2022
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Balance beginning $ 2,982 $ 0 $ 0  
Change in fair value (227) $ 0 (13,403) $ 0
Balance ending $ 2,755   $ 2,982 $ 0