Schedule of Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income (Loss)
Net Gain (Loss) on Sales of Securities	$ (5.4)	$ 6.7	$ (11.6)	$ 2.9
Other-Than-Temporary Impairment Loss on Fixed Maturity Securities	(20.8)	0.0	(20.8)	(1.0)
Net Investment Income	599.4	619.2	1,819.0	1,845.1
Derivative, Gain (Loss) on Derivative, Net	(0.1)	0.2	(0.7)	0.9
Interest and Debt Expense	(46.1)	(42.7)	(130.8)	(125.3)
Other Expenses	(227.7)	(212.9)	(695.5)	(657.9)
Income Before Income Tax	299.4	(377.8)	1,004.7	318.1
Total Income Tax Expense (Benefit)	57.4	(93.1)	200.6	43.8
Net Income (Loss)	242.0	(284.7)	804.1	274.3
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	0.0	1.2	(16.8)	1.3
Other-Than-Temporary Impairment Loss on Fixed Maturity Securities	(20.8)	0.0	(20.8)	(1.0)
Income Before Income Tax	(20.8)	1.2	(37.6)	0.3
Total Income Tax Expense (Benefit)	(4.4)	0.3	(7.9)	0.1
Net Income (Loss)	(16.4)	0.9	(29.7)	0.2
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	22.9	15.1	59.3	44.5
Total Income Tax Expense (Benefit)	4.9	3.1	12.5	9.3
Net Income (Loss)	18.0	12.0	46.8	35.2
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	19.0	16.4	54.0	48.1
Derivative, Gain (Loss) on Derivative, Net	4.4	0.0	8.8	0.2
Interest and Debt Expense	(0.6)	(0.6)	(1.6)	(1.6)
Reclassification out of Accumulated Other Comprehensive Income or Loss | Net Gain on Hedges | Forwards
Reclassification Adjustment out of Accumulated Other Comprehensive Income (Loss)
Interest and Debt Expense	(0.6)	0.0	(0.6)	0.0
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)	(0.3)	(0.9)
Derivative, Gain (Loss) on Derivative, Net	0.8	(0.4)	(1.0)	(1.3)
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)	(13.9)	(16.8)
Reclassification out of Accumulated Other Comprehensive Income or Loss | Prior Service Credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income (Loss)
Other Expenses	0.1	0.1	0.2	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 (Loss)
Income Before Income Tax	(4.5)	(5.5)	(13.7)	(16.6)
Total Income Tax Expense (Benefit)	(0.4)	(1.1)	(2.4)	(3.6)
Net Income (Loss)	$ (4.1)	$ (4.4)	$ (11.3)	$ (13.0)