Schedule of Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Investment Income	$ 613.2	$ 599.4	$ 1,767.2	$ 1,819.0
Interest and Debt Expense	(49.4)	(46.1)	(142.6)	(130.8)
Other Expenses	(232.3)	(227.7)	(724.2)	(695.5)
Income Before Income Tax	299.6	299.4	839.3	1,004.7
Income Tax Expense (Benefit)	68.5	57.4	181.7	200.6
Net Income	231.1	242.0	657.6	804.1
Reclassification out of Accumulated Other Comprehensive Income | Net Unrealized Gain on Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other-Than-Temporary Impairment Loss	0.0	(20.8)	(59.3)	(20.8)
Gain (Loss) on Investments, Excluding Other than Temporary Impairments	(0.1)	0.0	(0.1)	(16.8)
Income Before Income Tax	(0.1)	(20.8)	(59.4)	(37.6)
Income Tax Expense (Benefit)	0.0	(4.4)	(12.5)	(7.9)
Net Income	(0.1)	(16.4)	(46.9)	(29.7)
Reclassification out of Accumulated Other Comprehensive Income | Net Gain on Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Income Before Income Tax	18.5	22.9	56.5	59.3
Income Tax Expense (Benefit)	3.9	4.9	11.9	12.5
Net Income	14.6	18.0	44.6	46.8
Reclassification out of Accumulated Other Comprehensive Income | Net Gain on Hedges | Interest Rate Swaps
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Gain (Loss) on Investments, Excluding Other than Temporary Impairments	0.0	4.4	0.4	8.8
Net Investment Income	18.2	19.0	55.9	54.0
Interest and Debt Expense	(0.4)	(0.6)	(1.5)	(1.6)
Reclassification out of Accumulated Other Comprehensive Income | Net Gain on Hedges | Foreign Exchange Contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Gain (Loss) on Investments, Excluding Other than Temporary Impairments	0.2	0.8	0.2	(1.0)
Net Investment Income	0.5	(0.1)	1.5	(0.3)
Reclassification out of Accumulated Other Comprehensive Income | Net Gain on Hedges | Forwards
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Interest and Debt Expense	0.0	(0.6)	0.0	(0.6)
Reclassification out of Accumulated Other Comprehensive Income | Net Actuarial Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses	(4.9)	(4.6)	(14.8)	(13.9)
Reclassification out of Accumulated Other Comprehensive Income | Prior Service Credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses	0.1	0.1	0.2	0.2
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	(4.8)	(4.5)	(14.6)	(13.7)
Income Tax Expense (Benefit)	(0.9)	(0.4)	(3.1)	(2.4)
Net Income	$ (3.9)	$ (4.1)	$ (11.5)	$ (11.3)