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, 2023	Nov. 15, 2023	Dec. 31, 2022	Dec. 31, 2021
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	$ 2,697.2
Total reinsurance recoverables on unpaid losses and LAE	2,797.4
Unallocated loss adjustment expenses	70.7
Unamortized reserve discount	(20.8)		$ (19.3)	$ (18.8)
Reserves for losses and loss adjustment expenses	5,544.5	$ 5,526.4	5,051.6	5,595.0
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	1,546.4
Total reinsurance recoverables on unpaid losses and LAE	1,382.6
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	532.0
Total reinsurance recoverables on unpaid losses and LAE	424.4
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	43.3
Total reinsurance recoverables on unpaid losses and LAE	169.9
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	55.9
Total reinsurance recoverables on unpaid losses and LAE	25.5
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	88.3
Total reinsurance recoverables on unpaid losses and LAE	246.3
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	113.9
Total reinsurance recoverables on unpaid losses and LAE	181.7
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	147.7
Total reinsurance recoverables on unpaid losses and LAE	53.2
Unamortized reserve discount	(4.7)		$ (4.6)	$ (4.7)
Other Lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	169.7
Total reinsurance recoverables on unpaid losses and LAE	$ 313.8