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, 2021	Dec. 31, 2020	Dec. 31, 2019
Short-duration Insurance Contracts, Discounted Liabilities [Line Items]
Carrying Amount of Reserves for Losses & LAE	$ 277.4	$ 278.8	$ 302.0
Aggregate Amount of Discount	18.8	17.8	17.9
Interest Accretion	1.1	1.9	1.5
U.S. Operations | Specialty Admitted | Liability
Short-duration Insurance Contracts, Discounted Liabilities [Line Items]
Carrying Amount of Reserves for Losses & LAE	163.1	150.4	153.1
Aggregate Amount of Discount	14.1	12.9	13.0
Interest Accretion	$ 0.9	$ 1.9	$ 1.3
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	$ 114.3	$ 128.4	$ 148.9
Aggregate Amount of Discount	4.7	4.9	4.9
Interest Accretion	$ 0.2	$ 0.0	$ 0.2
Discount Rate	3.50%	3.50%	3.50%