Reinsurance - Premium Income Data (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Reinsurance Disclosures [Abstract]
Direct Premium Income	$ 10,732.1	$ 10,286.8	$ 9,893.6
Reinsurance Assumed	69.8	80.4	78.7
Reinsurance Ceded	(304.5)	(321.2)	(355.8)
Premium Income	10,497.4	10,046.0	9,616.5
Policyholder Benefits and Claims Incurred, Direct	8,027.9	8,001.8	8,205.3
Policyholder Benefits and Claims Incurred, Assumed	152.5	138.7	135.1
Ceded Benefits and Change in Reserves for Future Benefits	(700.2)	(828.6)	(798.3)
Policy Benefits	7,480.2	7,311.9	7,542.1
Liability for Future Policy Benefit, Remeasurement Gain (Loss), Direct	(546.8)	(49.2)	(372.5)
Liability for Future Policy Benefit, Remeasurement Gain (Loss), Assumed	2.2	(0.1)	36.4
Liability for Future Policy Benefit, Remeasurement Gain (Loss), Ceded	(17.7)	(5.5)	(211.4)
Liability for Future Policy Benefit, Remeasurement Gain (Loss)	$ (562.3)	$ (54.8)	$ (547.5)