Schedule of Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Investment Gain (Loss)	$ (36.0)	$ (15.7)	$ 76.7
Net Investment Income	2,096.7	2,122.2	2,213.2
Other Expenses	(1,112.0)	(1,006.7)	(999.3)
Income Tax Expense (Benefit)	356.3	342.8	279.6
Net Income	1,283.8	1,407.2	981.0
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)	(48.7)	(24.8)	60.8
Other-Than-Temporary Impairment Loss	(2.2)	(4.6)	(9.3)
Income Before Income Tax	(50.9)	(29.4)	51.5
Income Tax Expense (Benefit)	(10.9)	(6.2)	10.8
Net Income	(40.0)	(23.2)	40.7
Reclassification out of Accumulated Other Comprehensive Income | Net Gain on Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Income Before Income Tax	33.8	51.9	64.2
Income Tax Expense (Benefit)	7.1	10.9	13.5
Net Income	26.7	41.0	50.7
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	0.0	2.0
Net Investment Income	34.7	51.1	60.6
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)	(0.4)	1.8	(0.1)
Net Investment Income	(0.5)	(1.0)	1.7
Reclassification out of Accumulated Other Comprehensive Income | Net Actuarial Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses	(7.2)	(15.7)	(22.6)
Reclassification out of Accumulated Other Comprehensive Income | Prior Service Credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses	0.2	0.2	0.2
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	(7.0)	(15.5)	(22.4)
Income Tax Expense (Benefit)	(1.7)	(3.3)	(4.7)
Net Income	$ (5.3)	$ (12.2)	$ (17.7)