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, 2024	Dec. 31, 2022
Short-duration Insurance Contracts, Discounted Liabilities [Line Items]
Carrying Amount of Reserves for Losses & LAE	$ 259.4		$ 279.0	$ 261.0
Aggregate Amount of Discount	20.8		22.0	19.3
Interest Accretion	0.1	$ 1.4	1.6	1.8
Casualty Lines
Short-duration Insurance Contracts, Discounted Liabilities [Line Items]
Carrying Amount of Reserves for Losses & LAE	182.6		197.1	168.0
Aggregate Amount of Discount	16.1		17.4	14.7
Interest Accretion	$ 0.1	1.3	$ 1.5	$ 1.7
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		$ 81.9	$ 93.0
Aggregate Amount of Discount	4.7		4.6	4.6
Interest Accretion	$ 0.0	$ 0.1	$ 0.1	$ 0.1
Discount Rate	3.50%		3.50%	3.50%