Loss and Loss Adjustment Expense Reserves - LAE Reserve Activity (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Loss and LAE incurred relating to:
Net incurred loss and LAE	$ 258.5	$ 296.9	$ 686.8	$ 666.6
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross beginning balance	2,288.5	1,890.1	2,127.5	1,605.1
Less: beginning reinsurance recoverable on unpaid losses	(468.5)	(458.5)	(434.4)	(340.8)
Net loss and LAE reserves	1,820.0	1,431.6	1,693.1	1,264.3
Loss and LAE incurred relating to:
Current year losses	254.7	313.1	722.1	676.0
Prior year losses	(0.1)	(24.8)	(70.0)	(32.0)
Net incurred loss and LAE	254.6	288.3	652.1	644.0
Loss and LAE paid relating to:
Current year losses	(33.3)	(19.1)	(146.0)	(27.4)
Prior year losses	(84.9)	(86.3)	(264.7)	(261.8)
Net paid loss and LAE	(118.2)	(105.4)	(410.7)	(289.2)
Foreign currency translation and other adjustments to loss and LAE reserves	(1.0)	11.3	20.9	6.7
Net ending balance	1,955.4	1,625.8	1,955.4	1,625.8
Plus: ending reinsurance recoverable on unpaid losses	(522.5)	(507.2)	(522.5)	(507.2)
Gross ending balance	$ 2,477.9	$ 2,133.0	$ 2,477.9	$ 2,133.0