Underwriting, Acquisition and Insurance Expenses (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Underwriting, Acquisition and Insurance Expenses [Abstract]
Commissions	$ 57.5	$ 77.9
Other underwriting and insurance expenses	86.0	103.2
Total underwriting, acquisition and insurance expenses before deferral	143.5	181.1
Net deferral of policy acquisition costs	(6.5)	(8.2)
Total underwriting, acquisition and insurance expenses	$ 137.0	$ 172.9