Reconciliation of Total Revenue and Income Before Income Tax to Operating Revenue and Operating Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting, Revenue Reconciling Item
Total Revenue	$ 12,887.3	$ 12,385.9	$ 11,984.1
Net Investment Loss	(34.6)	(36.0)	(15.7)
Income Before Income Tax	2,251.3	1,640.1	1,750.0
Closed Block Individual Disability Reinsurance Transaction
Segment Reporting, Revenue Reconciling Item
Amortization of the Cost of Reinsurance	(41.4)	(44.1)	(50.3)
Operating Segments
Segment Reporting, Revenue Reconciling Item
Total Revenue	12,887.3	12,385.9	11,984.1
Net Investment Loss	(34.6)	(36.0)	(15.7)
Adjusted Operating Revenue	12,921.9	12,421.9	11,999.8
Income Before Income Tax	2,251.3	1,640.1	1,750.0
Reserve Assumption Updates	357.4	(177.2)	243.3
Loss on Legal Settlement	15.3	0.0	0.0
Adjusted Operating Income	2,010.3	1,932.2	1,607.1
Operating Segments | Closed Block Individual Disability Reinsurance Transaction
Segment Reporting, Revenue Reconciling Item
Non-Contemporaneous Reinsurance	$ (25.1)	$ (34.8)	$ (34.4)