Reconciliation of Total Revenue and Income Before Income Tax to Operating Revenue and Operating Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated
Total Revenue	$ 3,217.0	$ 3,092.5	$ 9,650.7	$ 9,240.8
Net Investment Loss	(12.9)	(31.0)	(24.5)	(30.0)
Adjusted Operating Revenue	3,229.9	3,123.5	9,675.2	9,270.8
Income Before Income Tax	814.6	261.7	1,805.8	1,211.4
Reserve Assumption Updates	357.4	(177.2)	357.4	(177.2)
Litigation Settlement, Loss	(15.3)	0.0	(15.3)	0.0
Adjusted Operating Income	501.8	490.2	1,539.5	1,478.1
Closed Block Individual Disability Reinsurance Transaction
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated
Amortization of the Cost of Reinsurance	(10.4)	(11.1)	(31.1)	(33.1)
Non-Contemporaneous Reinsurance	$ (6.0)	$ (9.2)	$ (20.2)	$ (26.4)