Reinsurance - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Insurance [Abstract]
Reinsurance recoverables, allowance for doubtful accounts	$ 3.8	$ 4.1
Amount of collateral under reinsurance agreement	1,085.5	1,131.4
Paid loss recoverables in insurance recoverables	494.6	509.1
Losses and loss adjustment expenses	1,314.6	1,208.8	$ 1,220.7
Net of amounts ceded to reinsurers	$ 829.6	$ 941.3	$ 1,031.1