Reinsurance - Premium Income Data (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Reinsurance Disclosures [Abstract]
Direct Premium Income	$ 9,742.8	$ 9,621.9	$ 9,576.3
Reinsurance Assumed	90.6	94.1	116.5
Reinsurance Ceded	(352.4)	(337.9)	(327.2)
Premium Income	9,481.0	9,378.1	9,365.6
Ceded Benefits and Change in Reserves for Future Benefits	$ 862.5	$ 628.8	$ 650.1