Underwriting, Acquisition and Insurance Expenses (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Underwriting, Acquisition and Insurance Expenses [Abstract]
Commissions	$ 61.3	$ 71.7	$ 139.2	$ 148.2
Other underwriting and insurance expenses	97.1	104.0	200.3	210.6
Total underwriting, acquisition and insurance expenses before deferral	158.4	175.7	339.5	358.8
Net deferral of policy acquisition costs	2.6	1.6	(5.6)	(5.1)
Total underwriting, acquisition and insurance expenses	$ 161.0	$ 177.3	$ 333.9	$ 353.7