Schedule of Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Investment Gain (Loss)	$ (15.7)	$ 76.7	$ 1,199.1
Benefits and Change in Reserves for Future Benefits	(6,936.7)	(7,598.6)	(8,972.9)
Net Investment Income	2,122.2	2,213.2	2,360.7
Interest and Debt Expense	(188.5)	(185.0)	(188.2)
Other Expenses	(1,020.2)	(1,008.6)	(996.6)
Income Tax Expense (Benefit)	317.2	238.8	171.0
Net Income	1,314.2	824.2	793.0
Reclassification out of Accumulated Other Comprehensive Income | Net Unrealized Gain (Loss) on Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Investment Gain (Loss)	(24.8)	60.8	1,279.7
Other-Than-Temporary Impairment Loss	(4.6)	(9.3)	(53.6)
Benefits and Change in Reserves for Future Benefits	0.0	(133.1)	(1,284.5)
Income Before Income Tax	(29.4)	(81.6)	(58.4)
Income Tax Expense (Benefit)	(6.2)	(17.2)	(12.2)
Net Income	(23.2)	(64.4)	(46.2)
Reclassification out of Accumulated Other Comprehensive Income | Net Gain on Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Income Before Income Tax	51.9	64.2	106.7
Income Tax Expense (Benefit)	10.9	13.5	22.4
Net Income	41.0	50.7	84.3
Reclassification out of Accumulated Other Comprehensive Income | Net Gain on Hedges | Interest Rate Swaps
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Investment Gain (Loss)	0.0	2.0	32.0
Net Investment Income	51.1	60.6	74.1
Interest and Debt Expense	0.0	0.0	(1.5)
Reclassification out of Accumulated Other Comprehensive Income | Net Gain on Hedges | Foreign Exchange Contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Investment Gain (Loss)	1.8	(0.1)	0.1
Net Investment Income	(1.0)	1.7	2.0
Reclassification out of Accumulated Other Comprehensive Income | Net Actuarial Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses	(15.7)	(22.6)	(19.8)
Reclassification out of Accumulated Other Comprehensive Income | Prior Service Credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses	0.2	0.2	0.1
Reclassification out of Accumulated Other Comprehensive Income | Curtailment Gain
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses	0.0	0.0	(0.1)
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	(15.5)	(22.4)	(19.8)
Income Tax Expense (Benefit)	(3.3)	(4.7)	(4.2)
Net Income	$ (12.2)	$ (17.7)	$ (15.6)