LOSS AND LOSS ADJUSTMENT EXPENSE RESERVES - Reconciliation (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Insurance [Abstract]
Losses—net of reinsurance	$ 133.0	$ 106.6
LAE—net of reinsurance	24.6	15.2
Reinsurance recoverables on unpaid losses	79.6	18.9	$ 11.4	$ 0.8
Loss and loss adjustment expense reserves	$ 237.2	$ 140.7	$ 33.3	$ 1.6