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, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Segment Reporting, Revenue Reconciling Item
Total Revenue	$ 3,092.5	$ 2,959.1	$ 9,240.8	$ 8,979.3
Net Investment Loss	(31.0)	(4.4)	(30.0)	(22.3)
Operating Segments
Segment Reporting, Revenue Reconciling Item
Total Revenue	3,092.5	2,959.1	9,240.8	8,979.3
Net Investment Loss	(31.0)	(4.4)	(30.0)	(22.3)
Adjusted Operating Revenue	3,123.5	2,963.5	9,270.8	9,001.6
Income Before Income Tax	261.7	641.6	1,211.4	1,376.5
Reserve Assumption Updates	(177.2)	243.3	(177.2)	243.3
Adjusted Operating Income	490.2	416.2	1,478.1	1,218.1
Operating Segments | Closed Block Individual Disability Reinsurance Transaction
Segment Reporting, Revenue Reconciling Item
Amortization of the Cost of Reinsurance	(11.1)	(11.8)	(33.1)	(38.5)
Non-Contemporaneous Reinsurance	$ (9.2)	$ (1.7)	$ (26.4)	$ (24.1)