Reserves for Losses and Loss Adjustment Expenses - Schedule 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, 2024	Dec. 31, 2023	Nov. 15, 2023	Dec. 31, 2022
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	$ 2,957.6
Total reinsurance recoverables on unpaid losses and LAE	2,794.7
Unallocated LAE	68.3
Unamortized reserve discount	(22.0)	$ (20.8)		$ (19.3)
Reserves for losses and loss adjustment expenses	5,798.6	5,544.5	$ 5,526.4	5,051.6
Casualty Lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	1,313.5
Total reinsurance recoverables on unpaid losses and LAE	1,046.0
Unamortized reserve discount	(17.4)	(16.1)		(14.7)
Specialty Lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	309.3
Total reinsurance recoverables on unpaid losses and LAE	214.3
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	1,266.1
Total reinsurance recoverables on unpaid losses and LAE	1,042.2
Unamortized reserve discount	(4.6)	$ (4.7)		$ (4.6)
Other lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	68.7
Total reinsurance recoverables on unpaid losses and LAE	$ 492.2