Reinsurance - Premium Income Data (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Reinsurance Disclosures [Abstract]
Direct Premium Income	$ 9,900.5	$ 9,742.8	$ 9,621.9
Reinsurance Assumed	78.7	90.6	94.1
Reinsurance Ceded	(355.8)	(352.4)	(337.9)
Premium Income	9,623.4	9,481.0	9,378.1
Ceded Benefits and Change in Reserves for Future Benefits	$ 1,034.6	$ 862.5	$ 628.8