Schedule VI Supplemental Information for Property-Casualty Insurance Companies (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters [Abstract]
Deferred acquisition costs	$ 168.0	$ 163.6	$ 160.2
Reserves for losses and loss adjustment expenses	5,595.0	5,406.0	5,157.6
Unamortized discount in reserves for losses	18.8	17.8	17.9
Unearned premiums	1,466.8	1,464.8	1,410.9
Premiums earned	1,910.1	1,780.5	1,729.5
Net investment income	187.6	112.7	151.1
Current year	1,176.3	1,201.1	1,082.6
Prior years	138.3	7.7	138.1
Losses and loss adjustment expenses incurred	1,314.6	1,208.8	1,220.7
Amortization (Deferral) DAC	(15.8)	(9.3)	12.4
Paid losses and loss adjustment expenses, net of reinsurance	869.2	1,119.8	1,030.3
Gross premiums written	$ 3,181.2	$ 3,233.3	$ 3,130.2