Reconciliation of Total Revenue and Income Before Income Tax to Operating Revenue and Operating Income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Segment Reporting, Revenue Reconciling Item
Total Revenue	$ 2,982.5	$ 3,072.0
Net Investment Gain (Loss)	(13.8)	84.6
Closed Block Individual Disability Reinsurance Transaction
Segment Reporting, Revenue Reconciling Item
Amortization of the Cost of Reinsurance	16.7	20.0
Operating Segments
Segment Reporting, Revenue Reconciling Item
Total Revenue	2,982.5	3,072.0
Net Investment Gain (Loss)	(13.8)	84.6
Adjusted Operating Revenue	2,996.3	2,987.4
Income Before Income Tax	312.1	198.8
Adjusted Operating Income	342.6	273.5
Operating Segments | Closed Block Individual Disability Reinsurance Transaction
Segment Reporting, Revenue Reconciling Item
Net Investment Gain (Loss)	0.0	67.6
Change in Benefit Reserves and Transaction Costs	0.0	(139.3)
Amortization of the Cost of Reinsurance	(16.7)	(20.0)
Total Items Related to Closed Block Individual Disability Reinsurance Transaction	(16.7)	(159.3)
Operating Segments | Unrelated to Significant Transaction
Segment Reporting, Revenue Reconciling Item
Net Investment Gain (Loss)	$ (13.8)	$ 17.0