Schedule of Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended													12 Months Ended
	Dec. 31, 2020		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
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Benefits and Change in Reserves for Future Benefits														$ (8,972.9)	$ (7,496.2)	$ (8,020.4)
Net Investment Income	$ 593.5		$ 613.2		$ 569.0		$ 585.0	$ 616.3		$ 599.4		$ 624.9	$ 594.7	2,360.7	2,435.3	2,453.7
Interest and Debt Expense														(188.2)	(177.4)	(167.3)
Other Expenses														(996.6)	(943.6)	(891.2)
Income Before Income Tax	124.7	[1],[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
Income Tax Expense (Benefit)														171.0	281.8	104.4
Net Income	$ 135.4	[1],[2],[3],[4],[9]	$ 231.1	[5]	$ 265.5	[6]	$ 161.0	$ 296.2	[7]	$ 242.0	[8]	$ 281.2	$ 280.9	793.0	1,100.3	523.4
Reclassification out of Accumulated Other Comprehensive Income | Net Unrealized Gain (Loss) on Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Realized Investment Gain (Loss), Excluding Other-Than-Temporary Impairment Loss on Fixed Maturity Securities														1,279.7	(18.0)	(4.8)
Other-Than-Temporary Impairment Loss														(53.6)	(25.3)	(17.5)
Benefits and Change in Reserves for Future Benefits														(1,284.5)	0.0	0.0
Income Before Income Tax														(58.4)	(43.3)	(22.3)
Income Tax Expense (Benefit)														(12.2)	(9.1)	(4.7)
Net Income														(46.2)	(34.2)	(17.6)
Reclassification out of Accumulated Other Comprehensive Income | Net Gain on Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Income Before Income Tax														106.7	79.3	61.4
Income Tax Expense (Benefit)														22.4	16.7	12.9
Net Income														84.3	62.6	48.5
Reclassification out of Accumulated Other Comprehensive Income | Net Gain on Hedges | Interest Rate Swaps
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Realized Investment Gain (Loss), Excluding Other-Than-Temporary Impairment Loss on Fixed Maturity Securities														32.0	8.8	0.2
Net Investment Income														74.1	73.6	65.9
Interest and Debt Expense														(1.5)	(2.1)	(2.1)
Reclassification out of Accumulated Other Comprehensive Income | Net Gain on Hedges | Foreign Exchange Contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Realized Investment Gain (Loss), Excluding Other-Than-Temporary Impairment Loss on Fixed Maturity Securities														0.1	(1.3)	(1.5)
Net Investment Income														2.0	0.8	(1.1)
Reclassification out of Accumulated Other Comprehensive Income | Net Gain on Hedges | Forwards
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Interest and Debt Expense														0.0	(0.5)	0.0
Reclassification out of Accumulated Other Comprehensive Income | Net Actuarial Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses														(19.8)	(18.6)	(22.3)
Reclassification out of Accumulated Other Comprehensive Income | Prior Service Credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses														0.1	0.2	0.2
Reclassification out of Accumulated Other Comprehensive Income | Unrecognized Pension and Postretirement Benefit Costs
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Income Before Income Tax														(19.8)	(18.4)	(22.1)
Income Tax Expense (Benefit)														(4.2)	(4.0)	(4.8)
Net Income														(15.6)	(14.4)	(17.3)
Reclassification out of Accumulated Other Comprehensive Income | Curtailment Gain
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses														$ (0.1)	$ 0.0	$ 0.0

[1] 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.

[2] 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.

[3] 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.

[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.

[9] Fourth quarter of 2020 net tax benefit from the Closed Block individual disability reinsurance transaction of $36.5 million.