Schedule of Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income (Loss)
Net Gain (Loss) on Sales of Securities and Other Invested Assets	$ (7.3)	$ (2.6)	$ (6.2)	$ (3.8)
Other-Than-Temporary Impairment Loss on Fixed Maturity Securities	0.0	0.0	0.0	1.0
Net Investment Income	624.9	623.6	1,219.6	1,225.9
Derivative, Gain (Loss) on Derivative, Net	(0.9)	0.0	(0.6)	0.7
Interest and Debt Expense	(42.6)	(42.4)	(84.7)	(82.6)
Other Expenses	(229.9)	(220.8)	(467.8)	(445.0)
Income Before Income Tax	352.0	354.8	705.3	695.9
Total Income Tax Expense (Benefit)	70.8	69.3	143.2	136.9
Net Income	281.2	285.5	562.1	559.0
Reclassification out of Accumulated Other Comprehensive Income or Loss | Net Unrealized Gain (Loss) on Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income (Loss)
Net Gain (Loss) on Sales of Securities and Other Invested Assets	(8.1)	(0.1)	(16.8)	0.1
Other-Than-Temporary Impairment Loss on Fixed Maturity Securities	0.0	0.0	0.0	1.0
Income Before Income Tax	(8.1)	(0.1)	(16.8)	(0.9)
Total Income Tax Expense (Benefit)	(1.7)	0.0	(3.5)	(0.2)
Net Income	(6.4)	(0.1)	(13.3)	(0.7)
Reclassification out of Accumulated Other Comprehensive Income or Loss | Net Gain on Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income (Loss)
Income Before Income Tax	16.4	14.7	36.4	29.4
Total Income Tax Expense (Benefit)	3.4	3.2	7.6	6.2
Net Income	13.0	11.5	28.8	23.2
Reclassification out of Accumulated Other Comprehensive Income or Loss | Net Gain on Hedges | Interest Rate Swaps
Reclassification Adjustment out of Accumulated Other Comprehensive Income (Loss)
Net Investment Income	17.6	16.1	35.0	31.7
Derivative, Gain (Loss) on Derivative, Net	0.4	0.3	4.4	0.2
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 (Loss)
Net Investment Income	(0.1)	(0.3)	(0.2)	(0.6)
Derivative, Gain (Loss) on Derivative, Net	(1.0)	(0.9)	(1.8)	(0.9)
Reclassification out of Accumulated Other Comprehensive Income or Loss | Net Actuarial Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income (Loss)
Other Expenses	(4.6)	(5.6)	(9.3)	(11.2)
Reclassification out of Accumulated Other Comprehensive Income or Loss | Prior Service Credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income (Loss)
Other Expenses	0.0	0.0	0.1	0.1
Reclassification out of Accumulated Other Comprehensive Income or Loss | Unrecognized Pension and Postretirement Benefit Costs
Reclassification Adjustment out of Accumulated Other Comprehensive Income (Loss)
Income Before Income Tax	(4.6)	(5.6)	(9.2)	(11.1)
Total Income Tax Expense (Benefit)	(1.0)	(1.3)	(2.0)	(2.5)
Net Income	$ (3.6)	$ (4.3)	$ (7.2)	$ (8.6)