Underwriting, Acquisition and Insurance Expenses (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Underwriting Acquisition And Insurance Expenses [Abstract]
Commissions	$ 289.5	$ 268.0	$ 241.3
Other underwriting and insurance expenses	428.6	409.0	412.3
Underwriting, acquisition and insurance expenses, total	718.1	677.0	653.6
Net deferral of policy acquisition costs	(15.8)	(9.3)	12.4
Total underwriting, acquisition and insurance expenses	$ 702.3	$ 667.7	$ 666.0