Schedule VI Supplemental Information for Property-Casualty Insurance Companies (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters [Abstract]
Deferred acquisition costs	$ 107.0	$ 168.0	$ 163.6
Reserves for losses and loss adjustment expenses	5,051.6	5,595.0	5,406.0
Unamortized discount in reserves for losses	19.3	18.8	17.8
Unearned premiums	1,039.9	1,466.8	1,464.8
Premiums earned	1,740.4	1,910.1	1,780.5
Net investment income	129.8	187.6	112.7
Current year	1,102.2	1,176.3	1,201.1
Prior years	64.7	138.3	7.7
Losses and loss adjustment expenses incurred	1,166.9	1,314.6	1,208.8
Amortization (Deferral) DAC	(10.5)	(15.8)	(9.3)
Paid losses and loss adjustment expenses, net of reinsurance	999.5	869.2	1,119.8
Gross premiums written	$ 2,848.1	$ 3,181.2	$ 3,233.3