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, 2019	Dec. 31, 2018	Dec. 31, 2017
Short-duration Insurance Contracts, Discounted Liabilities [Line Items]
Carrying Amount of Reserves for Losses & LAE	$ 302.0	$ 303.9	$ 302.2
Aggregate Amount of Discount	17.9	16.9	17.6
Interest Accretion	1.5	3.4	4.0
U.S. Operations [Member] | Specialty Admitted [Member] | Liability [Member]
Short-duration Insurance Contracts, Discounted Liabilities [Line Items]
Carrying Amount of Reserves for Losses & LAE	153.1	140.8	126.7
Aggregate Amount of Discount	13.0	11.9	10.6
Interest Accretion	$ 1.3	$ 1.3	$ 1.9
Discount Rate	2.25%	2.25%	2.25%
Run Off Lines [Member]
Short-duration Insurance Contracts, Discounted Liabilities [Line Items]
Carrying Amount of Reserves for Losses & LAE	$ 148.9	$ 163.1	$ 175.5
Aggregate Amount of Discount	4.9	5.0	7.0
Interest Accretion	$ 0.2	$ 2.1	$ 2.1
Discount Rate	3.50%	3.50%	3.50%