Reconciliation of Total Revenue and Income Before Income Tax to Operating Revenue and Operating Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting, Revenue Reconciling Item
Total Revenue	$ 12,013.8	$ 13,162.1	$ 11,998.9
Net Investment Gain (Loss)	76.7	1,199.1	(23.2)
Income Before Income Tax	1,063.0	964.0	1,382.1
Capitalized Computer Software, Impairments	(12.1)	0.0	0.0
Operating Lease, Impairment Loss	(13.9)	(12.7)	0.0
Closed Block Individual Disability Reinsurance Transaction
Segment Reporting, Revenue Reconciling Item
Net Investment Gain (Loss)	67.6	1,302.3
Amortization of the Cost of Reinsurance	(79.1)	(2.6)
Operating Segments
Segment Reporting, Revenue Reconciling Item
Adjusted Operating Revenue	11,937.1	11,963.0	12,022.1
Total Revenue	12,013.8	13,162.1	11,998.9
Net Investment Gain (Loss)	76.7	1,199.1	(23.2)
Income Before Income Tax	1,063.0	964.0	1,382.1
Net Reserve Decrease Related to Reserve Assumption Updates, Before Tax	181.4	(169.0)	0.0
Costs Related to Early Retirement of Debt	(67.3)	0.0	(27.3)
Costs Related to Organizational Design Update		(23.3)
Adjusted Operating Income	1,116.6	1,278.0	1,432.6
Operating Segments | Closed Block Individual Disability Reinsurance Transaction
Segment Reporting, Revenue Reconciling Item
Net Investment Gain (Loss)	67.6	1,302.3	0.0
Change in Benefit Reserves and Transaction Costs	139.3	1,305.5	0.0
Amortization of the Cost of Reinsurance			0.0
Operating Segments | Unrelated to Significant Transaction
Segment Reporting, Revenue Reconciling Item
Net Investment Gain (Loss)	$ 9.1	$ (103.2)	$ (23.2)