Reconciliation of Total Revenue and Income Before Income Tax to Operating Revenue and Operating Income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Segment Reporting, Revenue Reconciling Item
Total Revenue	$ 3,200.3	$ 3,036.1
Net Investment Gain (Loss)	(1.2)	0.1
Operating Segments
Segment Reporting, Revenue Reconciling Item
Total Revenue	3,200.3	3,036.1
Net Investment Gain (Loss)	(1.2)	0.1
Adjusted Operating Revenue	3,201.5	3,036.0
Income Before Income Tax	495.7	451.3
Adjusted Operating Income	514.5	469.5
Operating Segments | Closed Block Individual Disability Reinsurance Transaction
Segment Reporting, Revenue Reconciling Item
Amortization of the Cost of Reinsurance	(10.4)	(11.0)
Non-Contemporaneous Reinsurance	$ (7.2)	$ (7.3)