Loss and Loss Adjustment Expense Reserves - Schedule of Ending Liabilities for Unpaid Loss and LAE, Net of Reinsurance (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	$ 1,972.7	$ 1,693.1	$ 1,264.3	$ 791.5
Reinsurance recoverable on unpaid losses	(508.3)	(434.4)	(340.8)	(505.0)
Loss and loss adjustment expense reserves	2,481.0	2,127.5	$ 1,605.1	$ 1,296.5
Ark Insurance Holdings Limited/Outrigger
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Reinsurance recoverable on unpaid losses	(508.3)	(434.4)
Reinsurance recoverables on paid losses	117.6	57.5
Ark Insurance Holdings Limited/Outrigger | Ark
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Reinsurance recoverable on unpaid losses	29.5	31.8
Reinsurance recoverables on paid losses	(5.2)	(3.1)
Property and Accident & Health | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	573.1	533.6
Total Reinsurance recoverables on unpaid losses	142.4	121.5
Marine & Energy | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	569.0	463.1
Total Reinsurance recoverables on unpaid losses	208.9	180.1
Specialty | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	508.2	417.0
Total Reinsurance recoverables on unpaid losses	44.4	51.3
Casualty-Active | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	230.6	178.0
Total Reinsurance recoverables on unpaid losses	113.0	80.7
Casualty-Runoff | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	57.1	66.5
Total Reinsurance recoverables on unpaid losses	(0.4)	0.8
WM Outrigger Re | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	$ 34.7	$ 34.9