Schedule of Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Investment Gain (Loss)	$ 0.1	$ (13.8)
Credit Losses on Fixed Maturity Securities	0.0	4.1
Net Investment Income	508.8	527.2
Other Expenses	(273.2)	(240.5)
Income Tax Expense (Benefit)	93.0	55.3
Net Income	358.3	240.4
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Net Unrealized Gain (Loss) on Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Investment Gain (Loss)	0.3	(8.5)
Credit Losses on Fixed Maturity Securities	0.0	(4.1)
Income Before Income Tax	0.3	(12.6)
Income Tax Expense (Benefit)	0.1	(2.6)
Net Income	0.2	(10.0)
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Net Gain on Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Income Before Income Tax	10.4	13.7
Income Tax Expense (Benefit)	2.2	2.9
Net Income	8.2	10.8
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Net Gain on Hedges | Interest Rate Swaps
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Investment Income	10.4	13.4
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	0.0	0.3
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Amortization of Net Actuarial Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses	(1.8)	(4.0)
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Amortization of 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	(1.7)	(3.9)
Income Tax Expense (Benefit)	(0.4)	(0.8)
Net Income	$ (1.3)	$ (3.1)