Reinsurance - Schedule of Reinsurance Premium (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Insurance [Abstract]
Direct written premiums	$ 2,990.6	$ 2,676.1	$ 2,510.4
Reinsurance ceded to other companies	(1,203.9)	(1,423.2)	(1,375.6)
Reinsurance assumed from other companies	190.6	557.2	619.8
Net written premiums	1,977.3	1,810.1	1,754.6
Direct earned premiums	2,917.7	2,660.6	2,412.4
Reinsurance ceded to other companies	(1,272.7)	(1,388.6)	(1,286.7)
Reinsurance assumed from other companies	265.1	508.5	604.0
Net earned premiums	$ 1,910.1	$ 1,780.5	$ 1,729.7
Percentage of reinsurance assumed to net earned premiums	13.90%	28.60%	34.90%