SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES, Warrant Liability (Q1) (Details) - USD ($) |
3 Months Ended | 7 Months Ended | 12 Months Ended | |
|---|---|---|---|---|
Mar. 31, 2021 |
Mar. 31, 2020 |
Dec. 31, 2019 |
Dec. 31, 2020 |
|
| Warrant Liability [Abstract] | ||||
| Change in fair value of warrant liability | $ (78,220,000) | $ 195,000 | $ 4,645,000 | $ (50,420,000) |