Reinsurance - Schedule of Reinsurance Premium (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Insurance [Abstract]
Direct written premiums		$ 2,682.8	$ 2,990.6	$ 2,676.1
Reinsurance ceded to other companies		(1,106.7)	(1,203.9)	(1,423.2)
Reinsurance assumed from other companies		165.4	190.6	557.2
Net written premiums		1,741.5	1,977.3	1,810.1
Direct earned premiums		2,757.8	2,917.7	2,660.6
Reinsurance ceded to other companies	$ (121.0)	(1,205.3)	(1,272.7)	(1,388.6)
Reinsurance assumed from other companies		187.9	265.1	508.5
Net earned premiums		$ 1,740.4	$ 1,910.1	$ 1,780.5
Percentage of reinsurance assumed to net earned premiums		10.80%	13.90%	28.60%