Reconciliation of Total Revenue and Income Before Income Tax to Operating Revenue and Operating Income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Segment Reporting, Revenue Reconciling Item
Total Revenue	$ 3,036.1	$ 2,980.6
Net Investment Gain (Loss)	0.1	(13.8)
Operating Segments
Segment Reporting, Revenue Reconciling Item
Total Revenue	3,036.1	2,980.6
Net Investment Gain (Loss)	0.1	(13.8)
Adjusted Operating Revenue	3,036.0	2,994.4
Income Before Income Tax	451.3	295.7
Adjusted Operating Income	469.5	335.3
Operating Segments | Closed Block Individual Disability Reinsurance Transaction
Segment Reporting, Revenue Reconciling Item
Amortization of the Cost of Reinsurance	(11.0)	(13.4)
Non-Contemporaneous Reinsurance	(7.3)	(12.4)
Operating Segments | Unrelated to Significant Transaction
Segment Reporting, Revenue Reconciling Item
Net Investment Gain (Loss)	$ 0.1	$ (13.8)