Reconciliation of Total Revenue and Income Before Income Tax to Operating Revenue and Operating Income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting, Revenue Reconciling Item
Total Revenue	$ 3,072.0	$ 2,871.1
Net Realized Investment Gain (Loss)	84.6	(144.0)
Income Before Income Tax	198.8	202.1
Closed Block Individual Disability Reinsurance Transaction
Segment Reporting, Revenue Reconciling Item
Amortization of the Cost of Reinsurance	20.0
Operating Segments
Segment Reporting, Revenue Reconciling Item
Total Revenue	3,072.0	2,871.1
Net Realized Investment Gain (Loss)	84.6	(144.0)
Adjusted Operating Revenue	2,987.4	3,015.1
Income Before Income Tax	198.8	202.1
Adjusted Operating Income	273.5	346.1
Operating Segments | Closed Block Individual Disability Reinsurance Transaction
Segment Reporting, Revenue Reconciling Item
Net Realized Investment Gain (Loss)	67.6	0.0
Change in Benefit Reserves and Transaction Costs	(139.3)	0.0
Amortization of the Cost of Reinsurance	(20.0)	0.0
Operating Segments | Unrelated to Significant Transaction
Segment Reporting, Revenue Reconciling Item
Net Realized Investment Gain (Loss)	$ 17.0	$ (144.0)