Schedule VI Supplemental Information for Property-Casualty Insurance Companies (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters [Abstract]
Deferred acquisition costs	$ 160.2	$ 167.3	$ 160.4
Reserves for losses and loss adjustment expenses	5,157.6	4,654.6	4,201.0
Unamortized discount in reserves for losses	17.9	16.9	17.6
Unearned premiums	1,410.9	1,300.9	1,207.7
Premiums earned	1,729.5	1,731.7	1,572.3
Net investment income	151.1	133.1	140.0
Current year	1,082.6	1,058.8	1,058.4
Prior years	138.1	(18.0)	(8.2)
Losses and loss adjustment expenses incurred	1,220.7	1,040.8	1,050.2
Net deferral of policy acquisition costs	12.4	(9.9)	(11.9)
Paid losses and loss adjustment expenses, net of reinsurance	1,030.3	938.9	889.4
Gross premiums written	$ 3,129.2	$ 2,955.2	$ 2,697.2