Reconciliation of Total Revenue and Income Before Income Tax to Operating Revenue and Operating Income (Detail) - USD ($) $ in Millions	3 Months Ended													12 Months Ended
	Dec. 31, 2020	[1]	Sep. 30, 2020		Jun. 30, 2020		Mar. 31, 2020	Dec. 31, 2019		Sep. 30, 2019		Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting, Revenue Reconciling Item
Total Revenue	$ 4,273.5		$ 2,996.3		$ 3,021.2		$ 2,871.1	$ 3,034.6		$ 2,960.0		$ 3,016.7	$ 2,987.6	$ 13,162.1	$ 11,998.9	$ 11,598.5
Net Realized Investment Gain (Loss)	1,304.9		4.4		33.8		(144.0)	9.2		(26.2)		(7.3)	1.1	1,199.1	(23.2)	(39.5)
Income Before Income Tax	$ 124.7	[2],[3],[4]	$ 299.6	[5]	$ 337.6	[6]	$ 202.1	$ 377.4	[7]	$ 299.4	[8]	$ 352.0	$ 353.3	964.0	1,382.1	627.8
Operating Lease, Impairment Loss														(12.7)
Closed Block Individual Disability Reinsurance Transaction
Segment Reporting, Revenue Reconciling Item
Net Realized Investment Gain (Loss)														1,302.3
Amortization of the Cost of Reinsurance														(2.6)
Operating Segments
Segment Reporting, Revenue Reconciling Item
Adjusted Operating Revenue														11,963.0	12,022.1	11,638.0
Total Revenue														13,162.1	11,998.9	11,598.5
Net Realized Investment Gain (Loss)														1,199.1	(23.2)	(39.5)
Income Before Income Tax														964.0	1,382.1	627.8
Long-term Care Reserve Increase														(151.5)	0.0	(750.8)
Reserve Increase for Group Pension, Before Taxes														(17.5)	0.0	0.0
Operating Lease, Impairment Loss														(12.7)	0.0	0.0
Costs Related to Organizational Design Update														(23.3)
Costs Related to Early Retirement of Debt														0.0	(27.3)	0.0
Adjusted Operating Income														1,278.0	1,432.6	1,418.1
Operating Segments | Closed Block Individual Disability Reinsurance Transaction
Segment Reporting, Revenue Reconciling Item
Net Realized Investment Gain (Loss)														1,302.3	0.0	0.0
Change in Benefit Reserves and Transaction Costs														(1,305.5)	0.0	0.0
Amortization of the Cost of Reinsurance														(2.6)	0.0	0.0
Operating Segments | Unrelated to Significant Transaction
Segment Reporting, Revenue Reconciling Item
Net Realized Investment Gain (Loss)														$ (103.2)	$ (23.2)	$ (39.5)

[1] Fourth quarter of 2020 net realized investment gain of $1,302.3 million before tax and $1,028.8 million after tax, related to the transfer of investments for the Closed Block individual disability reinsurance transaction.

[2] Fourth quarter of 2020 amortization of the cost of reinsurance related to the Closed Block individual disability reinsurance transaction of $2.6 million before tax and $2.0 million after tax.

[3] Fourth quarter of 2020 impacts from the Closed Block individual disability reinsurance transaction of $1,305.5 million before tax and $1,031.3 million after tax.

[4] Fourth quarter of 2020 reserve increases of $151.5 million and $17.5 million before tax and $119.7 million and $13.8 million after tax related to long-term care and group pension, respectively.

[5] Third quarter of 2020 costs related to organizational design update of $23.3 million before tax and $18.6 million after tax.

[6] Second quarter of 2020 impairment on ROU asset related to an operating lease for office space that we do not plan to continue using to support our general operations of $12.7 million before tax and $10.0 million after tax.

[7] Fourth quarter of 2019 cost related to the early retirement of debt of $2.1 million before tax and $1.7 million after tax.

[8] Third quarter of 2019 cost related to the early retirement of debt of $25.2 million before tax and $19.9 million after tax.