Underwriting, Acquisition and Insurance Expenses (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Underwriting Acquisition And Insurance Expenses [Abstract]
Commissions	$ 280.2	$ 289.5	$ 268.0
Other underwriting and insurance expenses	401.0	428.6	409.0
Underwriting, acquisition and insurance expenses, total	681.2	718.1	677.0
Net deferral of policy acquisition costs	(10.5)	(15.8)	(9.3)
Total underwriting, acquisition and insurance expenses	$ 670.7	$ 702.3	$ 667.7