Schedule of Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Investment Gain (Loss)	$ (1.2)	$ 0.1
Net Investment Income	513.5	508.8
Other Expenses	(291.1)	(273.2)
Income Tax Expense (Benefit)	100.5	93.0
Net Income	395.2	358.3
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Net Unrealized Loss on Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Investment Gain (Loss)	(15.8)	0.3
Income Tax Expense (Benefit)	(3.4)	0.1
Net Income	(12.4)	0.2
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Net Gain (Loss) on Derivatives
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Income Before Income Tax	5.6	10.4
Income Tax Expense (Benefit)	1.2	2.2
Net Income	4.4	8.2
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Net Gain (Loss) on Derivatives | Interest Rate Swaps
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Investment Income	5.8	10.4
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Net Gain (Loss) on Derivatives | Foreign Exchange Contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Investment Gain (Loss)	(0.1)	0.0
Net Investment Income	(0.1)	0.0
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Amortization of Net Actuarial Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses	(4.2)	(1.8)
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	(4.1)	(1.7)
Income Tax Expense (Benefit)	(0.9)	(0.4)
Net Income	$ (3.2)	$ (1.3)