Reconciliation of Total Revenue and Income Before Income Tax to Operating Revenue and Operating Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Segment Reporting, Revenue Reconciling Item
Total Revenue	$ 13,075.5	$ 12,887.3	$ 12,385.9
Net Investment Loss	(106.6)	(34.6)	(36.0)
Amortization of Deferred Gain on Reinsurance	9.0	0.0	0.0
Income Before Income Tax	933.5	2,251.3	1,640.1
Settlement Loss on the U.S. Pension Plan Annuity Purchase	(103.8)	0.0	0.0
Accelerated Charitable Contribution	(20.0)	0.0	0.0
Loss on Legal Settlement		15.3
UNITED STATES | Pension Plan
Segment Reporting, Revenue Reconciling Item
Settlement Loss on the U.S. Pension Plan Annuity Purchase	(103.8)	0.0	0.0
Operating Segments
Segment Reporting, Revenue Reconciling Item
Total Revenue	13,075.5	12,887.3	12,385.9
Net Investment Loss	(106.6)	(34.6)	(36.0)
Amortization of Deferred Gain on Reinsurance	9.0	0.0	0.0
Adjusted Operating Revenue	13,173.1	12,921.9	12,421.9
Income Before Income Tax	933.5	2,251.3	1,640.1
Reserve Assumption Updates	(478.5)	357.4	(177.2)
Loss on Legal Settlement	0.0	15.3	0.0
Adjusted Operating Income	1,779.7	2,010.3	1,932.2
Operating Segments | Closed Block Individual Disability Reinsurance Transaction
Segment Reporting, Revenue Reconciling Item
Non-Contemporaneous Reinsurance	(29.6)
Operating Segments | Long-term Care and Individual Disability Reinsurance Transaction
Segment Reporting, Revenue Reconciling Item
Net Investment Loss	(46.8)	0.0	0.0
Operating Segments | Unrelated to Significant Transaction
Segment Reporting, Revenue Reconciling Item
Net Investment Loss	$ (59.8)	$ (34.6)	$ (36.0)