Reconciliation of Total Revenue and Income Before Income Tax to Operating Revenue and Operating Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting, Revenue Reconciling Item
Total Revenue	$ 2,969.7	$ 2,996.3	$ 9,034.7	$ 8,888.6
Net Realized Investment Gain (Loss)	(0.1)	4.4	85.4	(105.8)
Net Realized Investment Gain (Loss)	(0.1)	4.4	85.4	(105.8)
Capitalized Computer Software, Impairments	12.1	0.0	12.1	0.0
Impairment Loss on ROU Asset	0.0		(13.9)	(12.7)
Costs Related to Organizational Design Update		23.3
Closed Block Individual Disability Reinsurance Transaction
Segment Reporting, Revenue Reconciling Item
Amortization of Prepaid Reinsurance Premium	(19.7)		(59.4)
Operating Segments
Segment Reporting, Revenue Reconciling Item
Total Revenue	2,969.7	2,996.3	9,034.7	8,888.6
Net Realized Investment Gain (Loss)	(0.1)	4.4	85.4	(105.8)
Adjusted Operating Revenue	2,969.8	2,991.9	8,949.3	8,994.4
Income Before Income Tax	409.9	299.6	871.3	839.3
Net Realized Investment Gain (Loss)	(0.1)	4.4	85.4	(105.8)
Net Reserve Decrease Related to Reserve Assumption Updates, Before Tax	181.4	0.0	181.4	0.0
Cost Related to Early Retirement of Debt	0.0	0.0	(67.3)	0.0
Impairment Loss on ROU Asset		0.0		(12.7)
Costs Related to Organizational Design Update				23.3
Adjusted Operating Income	260.4	318.5	896.5	981.1
Operating Segments | Closed Block Individual Disability Reinsurance Transaction
Segment Reporting, Revenue Reconciling Item
Net Realized Investment Gain (Loss)	0.0	0.0	67.6	0.0
Net Realized Investment Gain (Loss)	0.0	0.0	67.6	0.0
Change in Benefit Reserves and Transaction Costs	0.0	0.0	(139.3)	0.0
Amortization of Prepaid Reinsurance Premium	(19.7)	0.0	(59.4)	0.0
Operating Segments | Unrelated to Significant Transaction
Segment Reporting, Revenue Reconciling Item
Net Realized Investment Gain (Loss)	(0.1)	4.4	17.8	(105.8)
Net Realized Investment Gain (Loss)	$ (0.1)	$ 4.4	$ 17.8	$ (105.8)