Schedule of Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Realized Investment Gain, Excluding Other-Than-Temporary Impairment Loss on Fixed Maturity Securities	$ 9.8	$ 11.0	$ 28.9	$ 26.3
Other-Than-Temporary Impairment Loss	0.0	0.0	0.0	(30.5)
Net Investment Income	609.0	611.4	1,831.9	1,841.1
Interest and Debt Expense	(40.1)	(45.2)	(119.8)	(126.2)
Other Expenses	(193.5)	(205.5)	(630.5)	(618.9)
Income Before Income Tax	366.8	332.1	1,058.7	976.5
Total Income Tax	114.5	96.1	331.4	293.1
Net Income	252.3	236.0	727.3	683.4
Reclassification out of Accumulated Other Comprehensive Income or Loss | Net Unrealized Gain on Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Realized Investment Gain, Excluding Other-Than-Temporary Impairment Loss on Fixed Maturity Securities	8.2	1.1	13.8	14.8
Other-Than-Temporary Impairment Loss	0.0	0.0	0.0	(30.5)
Income Before Income Tax	8.2	1.1	13.8	(15.7)
Total Income Tax	3.0	0.2	4.9	(5.7)
Net Income	5.2	0.9	8.9	(10.0)
Reclassification out of Accumulated Other Comprehensive Income or Loss | Net Gain on Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Income Before Income Tax	9.0	13.0	35.1	41.1
Total Income Tax	3.1	4.5	12.3	14.3
Net Income	5.9	8.5	22.8	26.8
Reclassification out of Accumulated Other Comprehensive Income or Loss | Net Gain on Cash Flow Hedges | Interest Rate Swaps
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Realized Investment Gain, Excluding Other-Than-Temporary Impairment Loss on Fixed Maturity Securities	0.0	0.0	0.0	3.2
Net Investment Income	15.1	13.8	44.2	40.6
Interest and Debt Expense	(0.5)	(0.5)	(1.5)	(1.4)
Reclassification out of Accumulated Other Comprehensive Income or Loss | Net Gain on Cash Flow Hedges | Foreign Exchange Contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Realized Investment Gain, Excluding Other-Than-Temporary Impairment Loss on Fixed Maturity Securities	(5.2)	0.0	(6.7)	(0.5)
Net Investment Income	(0.4)	(0.3)	(0.9)	(0.8)
Reclassification out of Accumulated Other Comprehensive Income or Loss | Net Actuarial Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses	(5.1)	(4.1)	(15.2)	(12.2)
Reclassification out of Accumulated Other Comprehensive Income or Loss | Prior Service Credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses	0.2	0.0	0.5	0.2
Reclassification out of Accumulated Other Comprehensive Income or Loss | Unrecognized Pension and Postretirement Benefit Costs
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Income Before Income Tax	(4.9)	(4.1)	(14.7)	(12.0)
Total Income Tax	(1.7)	(1.5)	(5.1)	(4.3)
Net Income	$ (3.2)	$ (2.6)	$ (9.6)	$ (7.7)