Reconciliation of Total Revenue and Income Before Income Tax to Operating Revenue and Segment Adjusted Operating Income (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2026	Mar. 31, 2025	Dec. 31, 2025
Segment Reporting
Total Revenue	$ 3,355.2	$ 3,091.6
Net Investment Loss	(5.0)	(206.8)
Operating Segments
Segment Reporting
Total Revenue	3,355.2	3,091.6
Net Investment Loss	(5.0)	(206.8)
Segment Adjusted Operating Revenue	3,360.2	3,298.4
Income Before Income Tax	302.7	243.6
Segment Adjusted Operating Income	307.7	450.4
Operating Segments | Long-term Care and Individual Disability Reinsurance Transaction
Segment Reporting
Net Investment Loss	0.0	(175.9)	$ (46.8)
Operating Segments | Unrelated to Significant Transaction
Segment Reporting
Net Investment Loss	$ (5.0)	$ (30.9)