Schedule of Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended										12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Realized Investment Gain (Loss), Excluding Other-Than-Temporary Impairment Loss on Fixed Maturity Securities											$ 48.4	$ 54.7	$ (11.4)
Other-Than-Temporary Impairment Loss											(8.1)	(30.5)	(32.4)
Net Investment Income	$ 619.8		$ 609.0	$ 620.5	$ 602.4		$ 617.9	$ 611.4	$ 623.3	$ 606.4	2,451.7	2,459.0	2,481.2
Interest and Debt Expense											(159.9)	(166.0)	(152.8)
Other Expenses											(862.9)	(831.6)	(813.4)
Income Before Income Tax	345.3	[1]	366.8	361.6	330.3	[2]	371.2	332.1	340.2	304.2	1,404.0	1,347.7	1,238.3
Income Tax Expense (Benefit)											409.8	416.3	371.2
Net Income	$ 266.9	[1],[3]	$ 252.3	$ 245.1	$ 229.9	[2]	$ 248.0	$ 236.0	$ 236.8	$ 210.6	994.2	931.4	867.1
Reclassification out of Accumulated Other Comprehensive Income | Net Unrealized Gain 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											22.7	15.3	(12.2)
Other-Than-Temporary Impairment Loss											(8.1)	(30.5)	(32.4)
Income Before Income Tax											14.6	(15.2)	(44.6)
Income Tax Expense (Benefit)											5.1	(5.4)	(15.9)
Net Income											9.5	(9.8)	(28.7)
Reclassification out of Accumulated Other Comprehensive Income | Net Gain on Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Income Before Income Tax											46.8	53.3	83.4
Income Tax Expense (Benefit)											16.3	18.6	27.1
Net Income											30.5	34.7	56.3
Reclassification out of Accumulated Other Comprehensive Income | Net Gain on Cash Flow 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	3.2	0.5
Net Investment Income											59.0	54.5	50.2
Interest and Debt Expense											(2.0)	(1.9)	(1.8)
Reclassification out of Accumulated Other Comprehensive Income | Net Gain on Cash Flow 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											(9.2)	(1.4)	36.1
Net Investment Income											(1.2)	(1.1)	(1.6)
Reclassification out of Accumulated Other Comprehensive Income | Net Actuarial Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses											(20.3)	(16.3)	(11.9)
Reclassification out of Accumulated Other Comprehensive Income | Prior Service Credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses											0.7	0.5	0.3
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											(19.6)	(15.8)	(11.6)
Income Tax Expense (Benefit)											(6.7)	(5.5)	(4.0)
Net Income											$ (12.9)	$ (10.3)	$ (7.6)

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

[2] 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.

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