Loss and Loss Adjustment Expense Reserves - Schedule of Ending Liabilities for Unpaid Loss and LAE, Net of Reinsurance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
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	$ 1,693.1	$ 1,264.3	$ 791.5	$ 465.8
Reinsurance recoverable on unpaid losses	(434.4)	(340.8)	(505.0)	(428.9)
Loss and loss adjustment expense reserves	2,127.5	1,605.1	1,296.5	894.7
Ark Insurance Holdings Limited/Outrigger
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Reinsurance recoverable on unpaid losses	(434.4)	(340.8)
Ceded	200.9	55.7	278.5
Ark Insurance Holdings Limited/Outrigger | Ark Insurance Holdings Limited - Segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Reinsurance recoverable on unpaid losses	31.8	15.6
Ceded	(29.9)	(15.6)
Amount attributable to TPC Providers | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Reinsurance recoverable on unpaid losses			$ (145.4)	$ (276.8)
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	533.6	358.7
Total Reinsurance recoverables on unpaid losses	121.5	126.9
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	463.1	331.7
Total Reinsurance recoverables on unpaid losses	180.1	59.1
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	417.0	339.7
Total Reinsurance recoverables on unpaid losses	51.3	77.6
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	178.0	137.1
Total Reinsurance recoverables on unpaid losses	80.7	74.8
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	66.5	81.3
Total Reinsurance recoverables on unpaid losses	0.8	2.4
Other | 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	$ 34.9	$ 15.8