Schedule of Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other-Than-Temporary Impairment Loss on Fixed Maturity Securities	$ 0.0	$ 0.0	$ 1.0	$ 0.0
Net Investment Income	623.6	620.5	1,225.9	1,222.9
Derivative, Gain (Loss) on Derivative, Net	0.0	(0.3)	0.7	(0.5)
Interest and Debt Expense	(42.4)	(39.9)	(82.6)	(79.7)
Other Expenses	(220.8)	(204.3)	(445.0)	(433.1)
Income Before Income Tax	354.8	361.6	695.9	691.9
Total Income Tax Expense (Benefit)	69.3	116.5	136.9	216.9
Net Income	285.5	245.1	559.0	475.0
Reclassification out of Accumulated Other Comprehensive Income or Loss | Net Unrealized Gain on Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Gain (Loss) on Sales of Securities and Other Invested Assets	(0.1)	2.2	0.1	5.6
Other-Than-Temporary Impairment Loss on Fixed Maturity Securities	0.0	0.0	1.0	0.0
Income Before Income Tax	(0.1)	2.2	(0.9)	5.6
Total Income Tax Expense (Benefit)	0.0	0.8	(0.2)	1.9
Net Income	(0.1)	1.4	(0.7)	3.7
Reclassification out of Accumulated Other Comprehensive Income or Loss | Net Gain on Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Income Before Income Tax	14.7	13.8	29.4	26.1
Total Income Tax Expense (Benefit)	3.2	4.9	6.2	9.2
Net Income	11.5	8.9	23.2	16.9
Reclassification out of Accumulated Other Comprehensive Income or Loss | Net Gain on Hedges | Interest Rate Swaps
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Investment Income	16.1	14.8	31.7	29.1
Derivative, Gain (Loss) on Derivative, Net	0.3	0.0	0.2	0.0
Interest and Debt Expense	(0.5)	(0.5)	(1.0)	(1.0)
Reclassification out of Accumulated Other Comprehensive Income or Loss | Net Gain on Hedges | Foreign Exchange Contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Investment Income	(0.3)	(0.2)	(0.6)	(0.5)
Derivative, Gain (Loss) on Derivative, Net	(0.9)	(0.3)	(0.9)	(1.5)
Reclassification out of Accumulated Other Comprehensive Income or Loss | Net Actuarial Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses	(5.6)	(5.0)	(11.2)	(10.1)
Reclassification out of Accumulated Other Comprehensive Income or Loss | Prior Service Credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses	0.0	0.1	0.1	0.3
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	(5.6)	(4.9)	(11.1)	(9.8)
Total Income Tax Expense (Benefit)	(1.3)	(1.7)	(2.5)	(3.4)
Net Income	$ (4.3)	$ (3.2)	$ (8.6)	$ (6.4)