Loss and Loss Adjustment Expense Reserves - Schedule of Ending Liabilities for Unpaid Loss and LAE, Net of Reinsurance (Details) - Ark - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	$ 1,264.3	$ 791.5	$ 465.8	$ 262.6
Reinsurance recoverable on unpaid losses	(340.8)	(505.0)	(428.9)	(433.4)
Loss and loss adjustment expense reserves	1,605.1	1,296.5	894.7	696.0
Ark
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Ceded	(15.6)
Amount attributable to TPC Providers
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Reinsurance recoverable on unpaid losses	(0.0)	(145.4)	$ (276.8)	$ (319.2)
Property and Accident & Health
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	358.7	258.2
Total Reinsurance recoverables on unpaid losses	126.9	224.6
Specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	339.7	204.3
Total Reinsurance recoverables on unpaid losses	77.6	97.2
Marine & Energy
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	331.7	196.4
Total Reinsurance recoverables on unpaid losses	59.1	79.8
Casualty-Active
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	137.1	71.5
Total Reinsurance recoverables on unpaid losses	74.8	49.9
Casualty-Runoff
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	81.3	60.8
Total Reinsurance recoverables on unpaid losses	2.4	53.5
Other
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	$ 15.8	$ 0.3