Reconciliation of Total Revenue and Income Before Income Tax to Operating Revenue and Operating Income (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Segment Reporting, Revenue Reconciling Item
Total Revenue	$ 3,040.9	$ 2,993.0	$ 6,023.4	$ 6,065.0
Net Investment Gain (Loss)	(4.1)	0.9	(17.9)	85.5
Impairment Loss on ROU Asset		(13.9)
Operating Segments
Segment Reporting, Revenue Reconciling Item
Total Revenue	3,040.9	2,993.0	6,023.4	6,065.0
Net Investment Gain (Loss)	(4.1)	0.9	(17.9)	85.5
Adjusted Operating Revenue	3,045.0	2,992.1	6,041.3	5,979.5
Income Before Income Tax	443.1	262.6	755.2	461.4
Cost Related to Early Retirement of Debt	0.0	(67.3)	0.0	(67.3)
Impairment Loss on ROU Asset	0.0	(13.9)	0.0	(13.9)
Adjusted Operating Income	463.8	362.6	806.4	636.1
Operating Segments | Closed Block Individual Disability Reinsurance Transaction
Segment Reporting, Revenue Reconciling Item
Net Investment Gain (Loss)	0.0	0.0	0.0	67.6
Change in Benefit Reserves and Transaction Costs	0.0	0.0	0.0	(139.3)
Amortization of the Cost of Reinsurance	(16.6)	(19.7)	(33.3)	(39.7)
Operating Segments | Unrelated to Significant Transaction
Segment Reporting, Revenue Reconciling Item
Net Investment Gain (Loss)	$ (4.1)	$ 0.9	$ (17.9)	$ 17.9