Reserves for Losses and Loss Adjustment Expenses - Summary of Reconciliation of Net Incurred and Paid Development to Liability for Unpaid Losses and LAE in Consolidated Balance Sheets (Detail) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	$ 2,163.0
Total reinsurance recoverables on unpaid losses and LAE	2,838.5
Unallocated loss adjustment expenses	69.4
Unamortized reserve discount	(19.3)	$ (18.8)	$ (17.8)
Reserves for losses and loss adjustment expenses	5,051.6	5,595.0	5,406.0
U.S. Operations | Liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	1,219.8
Total reinsurance recoverables on unpaid losses and LAE	1,389.0
U.S. Operations | Professional
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	371.2
Total reinsurance recoverables on unpaid losses and LAE	356.9
U.S. Operations | Property
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	60.2
Total reinsurance recoverables on unpaid losses and LAE	205.2
U.S. Operations | Specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	45.3
Total reinsurance recoverables on unpaid losses and LAE	18.3
U.S. Operations | Specialty Admitted | Liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unamortized reserve discount	(14.7)	(14.1)	(12.9)
U.S. Operations | Specialty Admitted | Specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	45.3
International Operations | Argo Insurance Bermuda- Liability | Liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	78.8
Total reinsurance recoverables on unpaid losses and LAE	215.4
International Operations | Argo Insurance Bermuda- Liability | Professional
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	86.9
Total reinsurance recoverables on unpaid losses and LAE	137.0
Run-off Lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	168.0
Total reinsurance recoverables on unpaid losses and LAE	69.8
Unamortized reserve discount	(4.6)	$ (4.7)	$ (4.9)
Other Lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	132.8
Total reinsurance recoverables on unpaid losses and LAE	$ 446.9