Loss and Loss Adjustment Expense Reserves - Schedule of Ending Liabilities for Unpaid Loss and LAE, Net of Reinsurance (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total Reinsurance recoverables on unpaid losses	$ 428.9
Property and Accident & Health
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Loss and LAE reserves, net of reinsurance	175.0
Marine & Energy
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Loss and LAE reserves, net of reinsurance	99.3
Specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Loss and LAE reserves, net of reinsurance	85.2
Casualty - Active
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Loss and LAE reserves, net of reinsurance	37.4
Casualty - Runoff
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Loss and LAE reserves, net of reinsurance	68.4
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	465.8
Total Reinsurance recoverables on unpaid losses	428.9
Loss and loss adjustment expense reserves	894.7	$ 0.0
Ark | Amount attributable to TPC Providers
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total Reinsurance recoverables on unpaid losses	276.8
Ark | Property and Accident & Health
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Loss and LAE reserves, net of reinsurance	175.0
Total Reinsurance recoverables on unpaid losses	145.2
Ark | Marine & Energy
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Loss and LAE reserves, net of reinsurance	99.3
Total Reinsurance recoverables on unpaid losses	70.2
Ark | Specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Loss and LAE reserves, net of reinsurance	85.2
Total Reinsurance recoverables on unpaid losses	68.9
Ark | Casualty - Active
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Loss and LAE reserves, net of reinsurance	37.4
Total Reinsurance recoverables on unpaid losses	41.4
Ark | Casualty - Runoff
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Loss and LAE reserves, net of reinsurance	68.4
Total Reinsurance recoverables on unpaid losses	103.2
Ark | Other
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Loss and LAE reserves, net of reinsurance	$ 0.5