LOSS AND LOSS ADJUSTMENT EXPENSE RESERVES - Reconciliation (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Insurance [Abstract]
Loss and ALAE reserves—net of reinsurance	$ 220.5	$ 144.0
ULAE reserves—net of reinsurance	20.2	13.6
Reinsurance recoverables on unpaid losses	79.5	79.6	$ 18.9	$ 11.4
Loss and loss adjustment expense reserves	$ 320.2	$ 237.2	$ 140.7	$ 33.3