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, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated
Total Revenue	$ 3,361.4	$ 3,233.4	$ 6,453.0	$ 6,433.7
Net Investment Loss	(17.7)	(10.4)	(224.5)	(11.6)
Income Before Income Tax	417.0	495.5	660.6	991.2
Operating Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated
Total Revenue	3,361.4	3,233.4	6,453.0	6,433.7
Net Investment Loss	(17.7)	(10.4)	(224.5)	(11.6)
Adjusted Operating Revenue	3,379.1	3,243.8	6,677.5	6,445.3
Income Before Income Tax	417.0	495.5	660.6	991.2
Adjusted Operating Income	449.4	523.2	916.2	1,037.7
Long-term Care and Individual Disability Reinsurance Transaction | Operating Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated
Net Investment Loss	(8.5)	0.0	(184.4)	0.0
Closed Block Individual Disability Reinsurance Transaction | Operating Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated
Amortization of the Cost of Reinsurance	(9.7)	(10.3)	(19.3)	(20.7)
Non-Contemporaneous Reinsurance	(5.0)	(7.0)	(11.8)	(14.2)
Unrelated to Significant Transaction | Operating Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated
Net Investment Loss	$ (9.2)	$ (10.4)	$ (40.1)	$ (11.6)