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, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting, Revenue Reconciling Item
Total Revenue	$ 2,993.0	$ 3,021.2	$ 6,065.0	$ 5,892.3
Net Realized Investment Gain (Loss)	0.9	33.8	85.5	(110.2)
Income Before Income Tax	262.6	337.6	461.4	539.7
Net Realized Investment Gain (Loss)	0.9	33.8	85.5	(110.2)
Impairment Loss on ROU Asset	(13.9)			(12.7)
Closed Block Individual Disability Reinsurance Transaction
Segment Reporting, Revenue Reconciling Item
Amortization of Prepaid Reinsurance Premium	19.7		39.7
Operating Segments
Segment Reporting, Revenue Reconciling Item
Total Revenue	2,993.0	3,021.2	6,065.0	5,892.3
Net Realized Investment Gain (Loss)	0.9	33.8	85.5	(110.2)
Adjusted Operating Revenue	2,992.1	2,987.4	5,979.5	6,002.5
Income Before Income Tax	262.6	337.6	461.4	539.7
Net Realized Investment Gain (Loss)	0.9	33.8	85.5	(110.2)
Cost Related to Early Retirement of Debt	(67.3)	0.0	(67.3)	0.0
Impairment Loss on ROU Asset		(12.7)	(13.9)	(12.7)
Adjusted Operating Income	362.6	316.5	636.1	662.6
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	(39.7)	0.0
Operating Segments | Unrelated to Significant Transaction
Segment Reporting, Revenue Reconciling Item
Net Realized Investment Gain (Loss)	0.9	33.8	17.9	(110.2)
Net Realized Investment Gain (Loss)	$ 0.9	$ 33.8	$ 17.9	$ (110.2)