Schedule of Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended											12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018		Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other-Than-Temporary Impairment Loss												$ (17.5)	$ (8.1)	$ (30.5)
Net Investment Income	$ 608.6	$ 619.2		$ 623.6	$ 602.3	$ 619.8		$ 609.0	$ 620.5	$ 602.4		2,453.7	2,451.7	2,459.0
Interest and Debt Expense												(167.3)	(159.9)	(166.0)
Other Expenses												(891.2)	(855.0)	(824.6)
Income Before Income Tax	309.7	(377.8)	[1]	354.8	341.1	345.3	[2]	366.8	361.6	330.3	[3]	627.8	1,404.0	1,347.7
Income Tax Expense (Benefit)												104.4	409.8	416.3
Net Income	$ 249.1	$ (284.7)	[1]	$ 285.5	$ 273.5	$ 266.9	[2],[4]	$ 252.3	$ 245.1	$ 229.9	[3]	523.4	994.2	931.4
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												(4.8)	22.7	15.3
Other-Than-Temporary Impairment Loss												(17.5)	(8.1)	(30.5)
Income Before Income Tax												(22.3)	14.6	(15.2)
Income Tax Expense (Benefit)												(4.7)	5.1	(5.4)
Net Income												(17.6)	9.5	(9.8)
Reclassification out of Accumulated Other Comprehensive Income | Net Gain on Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Income Before Income Tax												61.4	46.8	53.3
Income Tax Expense (Benefit)												12.9	16.3	18.6
Net Income												48.5	30.5	34.7
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												0.2	0.2	3.2
Net Investment Income												65.9	59.0	54.5
Interest and Debt Expense												(2.1)	(2.0)	(1.9)
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												(1.5)	(9.2)	(1.4)
Net Investment Income												(1.1)	(1.2)	(1.1)
Reclassification out of Accumulated Other Comprehensive Income | Net Actuarial Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses												(22.3)	(20.3)	(16.3)
Reclassification out of Accumulated Other Comprehensive Income | Prior Service Credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses												0.2	0.7	0.5
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												(22.1)	(19.6)	(15.8)
Income Tax Expense (Benefit)												(4.8)	(6.7)	(5.5)
Net Income												$ (17.3)	$ (12.9)	$ (10.3)

[1] Third quarter of 2018 reserve increase of $750.8 million before tax and $593.1 million after tax related to long-term care.

[2] Fourth quarter of 2017 reserve increase of $39.0 million before tax and $25.4 million after tax related to unclaimed death benefits.

[3] First quarter of 2017 loss from a guaranty fund assessment related to an unaffiliated insurer that was declared insolvent of $20.6 million before tax and $13.4 million after tax.

[4] Fourth quarter of 2017 net tax benefit of $31.5 million related to the TCJA.