Loss and Loss Adjustment Expense Reserves - LAE Reserve Activity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Loss and LAE incurred relating to:
Net incurred loss and LAE	$ 888.5	$ 726.8	$ 536.4
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross beginning balance	1,605.1	1,296.5	894.7
Less: beginning reinsurance recoverable on unpaid losses	(340.8)	(505.0)	(428.9)
Net loss and LAE reserves	1,264.3	791.5	465.8
Loss and LAE incurred relating to:
Current Year	914.4	706.9	588.1
Prior Year	(58.6)	19.9	(51.7)
Net incurred loss and LAE	855.8	726.8	536.4
Loss and LAE paid relating to:
Current year losses	(109.2)	(51.3)	(98.9)
Prior year losses	(307.5)	(356.6)	(158.6)
Net paid loss and LAE	(416.7)	(407.9)	(257.5)
Change in TPC providers’ participation	0.0	145.4	57.5
Foreign currency translation and other adjustments to loss and LAE reserves	(10.3)	8.5	(10.7)
Net ending balance	1,693.1	1,264.3	791.5
Plus: ending reinsurance recoverable on unpaid losses	(434.4)	(340.8)	(505.0)
Gross ending balance	$ 2,127.5	1,605.1	1,296.5
P&C Insurance and Reinsurance (Ark/WM Outrigger) | 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)
Loss and LAE paid relating to:
Plus: ending reinsurance recoverable on unpaid losses			$ (145.4)