Loss and Loss Adjustment Expense Reserves - LAE Reserve Activity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Losses and LAE incurred relating to:
Net incurred losses and LAE	$ 726.8	$ 536.4	$ 314.8
Ark
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross beginning balance	1,296.5	894.7	696.0
Less: beginning reinsurance recoverable on unpaid losses	(505.0)	(428.9)	(433.4)
Net loss and LAE reserves	791.5	465.8	262.6
Losses and LAE incurred relating to:
Current Year	706.9	588.1	336.3
Prior Year	19.9	(51.7)	(21.5)
Net incurred losses and LAE	726.8	536.4	314.8
Loss and LAE paid relating to:
Current year losses	(51.3)	(98.9)	(43.9)
Prior year losses	(356.6)	(158.6)	(61.6)
Net paid losses and LAE	(407.9)	(257.5)	(105.5)
Change in TPC providers’ participation	145.4	57.5	(2.2)
Foreign currency translation and other adjustments to loss and LAE reserves	8.5	(10.7)	(3.9)
Net ending balance	1,264.3	791.5	465.8
Plus: ending reinsurance recoverable on unpaid losses	(340.8)	(505.0)	(428.9)
Gross ending balance	1,605.1	1,296.5	894.7
Ark | Amount attributable to TPC Providers
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Less: beginning reinsurance recoverable on unpaid losses	(145.4)	(276.8)	(319.2)
Loss and LAE paid relating to:
Plus: ending reinsurance recoverable on unpaid losses	$ (0.0)	$ (145.4)	$ (276.8)