Schedule of Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Benefits and Change in Reserves for Future Benefits	$ 2,051.2	$ 1,854.8
Net Investment Income	548.7	585.0
Interest and Debt Expense	(44.4)	(45.7)
Other Expenses	(245.0)	(235.6)
Income Before Income Tax	198.8	202.1
Income Tax Expense (Benefit)	45.8	41.1
Net Income	153.0	161.0
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Net Unrealized Gain on Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Gain (Loss) on Investments, Excluding Other than Temporary Impairments	68.3	0.0
Other-Than-Temporary Impairment Loss	(8.3)	(53.9)
Benefits and Change in Reserves for Future Benefits	(133.1)	0.0
Income Before Income Tax	(73.1)	(53.9)
Income Tax Expense (Benefit)	(15.4)	(11.3)
Net Income	(57.7)	(42.6)
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Net Gain on Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Income Before Income Tax	17.6	18.7
Income Tax Expense (Benefit)	3.7	3.8
Net Income	13.9	14.9
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Net Gain on Hedges | Interest Rate Swaps
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Gain (Loss) on Investments, Excluding Other than Temporary Impairments	1.8	0.0
Net Investment Income	15.3	18.8
Interest and Debt Expense	0.0	(0.6)
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Net Gain on Hedges | Foreign Exchange Contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Investment Income	0.5	0.5
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Net Actuarial Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses	(5.6)	(5.0)
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Prior Service Credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses	0.1	0.1
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Unrecognized Pension and Postretirement Benefit Costs
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Income Before Income Tax	(5.5)	(4.9)
Income Tax Expense (Benefit)	(1.2)	(1.0)
Net Income	$ (4.3)	$ (3.9)