Loss and Loss Adjustment Expense Reserves - Schedule of Ending Liabilities for Unpaid Loss and LAE, Net of Reinsurance (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total Reinsurance recoverables on unpaid losses	$ 505.0	$ 428.9
Ark
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses	791.5	465.8
Total Reinsurance recoverables on unpaid losses	505.0	428.9
Loss and loss adjustment expense reserves	1,296.5	894.7
Ark | Property and Accident & Health
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Loss and LAE reserves, net of reinsurance	258.2	175.0
Total Reinsurance recoverables on unpaid losses	224.6	145.2
Ark | Specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Loss and LAE reserves, net of reinsurance	204.3	85.2
Total Reinsurance recoverables on unpaid losses	97.2	68.9
Ark | Marine & Energy
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Loss and LAE reserves, net of reinsurance	196.4	99.3
Total Reinsurance recoverables on unpaid losses	79.8	70.2
Ark | Casualty - Active
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Loss and LAE reserves, net of reinsurance	71.5	37.4
Total Reinsurance recoverables on unpaid losses	49.9	41.4
Ark | Casualty - Runoff
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Loss and LAE reserves, net of reinsurance	60.8	68.4
Total Reinsurance recoverables on unpaid losses	53.5	103.2
Ark | Other
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Loss and LAE reserves, net of reinsurance	$ 0.3	$ 0.5