Reconciliation of Total Revenue and Income Before Income Tax to Operating Revenue and Operating Income (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Segment Reporting, Revenue Reconciling Item
Total Revenue	$ 3,233.4	$ 3,112.2	$ 6,433.7	$ 6,148.3
Net Investment Gain (Loss)	(10.4)	0.9	(11.6)	1.0
Income Before Income Tax	495.5	498.4	991.2	949.7
Operating Segments
Segment Reporting, Revenue Reconciling Item
Total Revenue	3,233.4	3,112.2	6,433.7	6,148.3
Net Investment Gain (Loss)	(10.4)	0.9	(11.6)	1.0
Adjusted Operating Revenue	3,243.8	3,111.3	6,445.3	6,147.3
Income Before Income Tax	495.5	498.4	991.2	949.7
Adjusted Operating Income	523.2	518.4	1,037.7	987.9
Operating Segments | Closed Block Individual Disability Reinsurance Transaction
Segment Reporting, Revenue Reconciling Item
Amortization of the Cost of Reinsurance	(10.3)	(11.0)	(20.7)	(22.0)
Non-Contemporaneous Reinsurance	$ (7.0)	$ (9.9)	$ (14.2)	$ (17.2)