Reinsurance - Premium Income Data (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021
Reinsurance Disclosures [Abstract]
Direct Premium Income	$ 10,286.8	$ 9,893.6		$ 9,736.8
Reinsurance Assumed	80.4	78.7		90.6
Reinsurance Ceded	(321.2)	(355.8)		(352.4)
Premium Income	10,046.0	9,616.5		9,475.0
Policyholder Benefits and Claims Incurred, Direct	8,001.8	8,205.3		8,817.1
Policyholder Benefits and Claims Incurred, Assumed	138.7	135.1		145.0
Ceded Benefits and Change in Reserves for Future Benefits	(828.6)	(798.3)		(848.8)
Policy Benefits	7,311.9	7,542.1	[1]	8,113.3	[1]
Liability for Future Policy Benefit, Remeasurement Gain (Loss), Direct	(49.2)	(372.5)		(546.2)
Liability for Future Policy Benefit, Remeasurement Gain (Loss), Assumed	(0.1)	36.4		1.9
Liability for Future Policy Benefit, Remeasurement Gain (Loss), Ceded	(5.5)	(211.4)		(15.6)
Liability for Future Policy Benefit, Remeasurement Gain (Loss)	$ (54.8)	$ (547.5)		$ (559.9)

[1] 1 Previously disclosed as Benefits and Change in Reserves for Future Benefits