Reserves for Losses and Loss Adjustment Expenses - Schedule of Information About Discounted Liabilities For Unpaid Losses and LAE (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Short-duration Insurance Contracts, Discounted Liabilities [Line Items]
Carrying Amount of Reserves for Losses & LAE	$ 261.0	$ 277.4	$ 278.8
Aggregate Amount of Discount	19.3	18.8	17.8
Interest Accretion	1.8	1.1	1.9
U.S. Operations | Specialty Admitted | Liability
Short-duration Insurance Contracts, Discounted Liabilities [Line Items]
Carrying Amount of Reserves for Losses & LAE	168.0	163.1	150.4
Aggregate Amount of Discount	14.7	14.1	12.9
Interest Accretion	$ 1.7	$ 0.9	$ 1.9
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	$ 93.0	$ 114.3	$ 128.4
Aggregate Amount of Discount	4.6	4.7	4.9
Interest Accretion	$ 0.1	$ 0.2	$ 0.0
Discount Rate	3.50%	3.50%	3.50%