Reserves for Losses and Loss Adjustment Expenses - Schedule of Information About Discounted Liabilities For Unpaid Losses and LAE (Detail) - USD ($) $ in Millions	2 Months Ended	11 Months Ended	12 Months Ended
	Dec. 31, 2023	Nov. 15, 2023	Dec. 31, 2022	Dec. 31, 2021
Short-duration Insurance Contracts, Discounted Liabilities [Line Items]
Carrying Amount of Reserves for Losses & LAE	$ 259.4		$ 261.0	$ 277.4
Aggregate Amount of Discount	20.8		19.3	18.8
Interest Accretion	0.1	$ 1.4	1.8	1.1
U.S. Operations | Specialty Admitted | Liability
Short-duration Insurance Contracts, Discounted Liabilities [Line Items]
Carrying Amount of Reserves for Losses & LAE	182.6		168.0	163.1
Aggregate Amount of Discount	16.1		14.7	14.1
Interest Accretion	$ 0.1	1.3	$ 1.7	$ 0.9
Discount Rate	2.25%		2.25%	2.25%
Run-off Lines
Short-duration Insurance Contracts, Discounted Liabilities [Line Items]
Carrying Amount of Reserves for Losses & LAE	$ 76.8		$ 93.0	$ 114.3
Aggregate Amount of Discount	4.7		4.6	4.7
Interest Accretion	$ 0.0	$ 0.1	$ 0.1	$ 0.2
Discount Rate	3.50%		3.50%	3.50%