Schedule of Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Investment Loss	$ (12.9)	$ (31.0)	$ (24.5)	$ (30.0)
Impairment Loss	(1.3)	0.0	(2.5)	0.0
credit losses	0.2	0.0	3.1	0.0
Net Investment Income	527.8	526.0	1,586.4	1,565.9
Other Expenses	311.5	271.8	903.3	818.6
Income Tax Expense (Benefit)	168.9	59.7	375.4	258.2
Net Income	645.7	202.0	1,430.4	953.2
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Net Unrealized Loss on Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Investment Loss	(7.4)	(37.0)	(28.6)	(47.0)
Impairment Loss	(1.3)	0.0	(2.5)	0.0
credit losses	(0.2)	0.0	(3.1)	0.0
Income Before Income Tax	(8.9)	(37.0)	(34.2)	(47.0)
Income Tax Expense (Benefit)	(2.1)	(7.7)	(7.4)	(9.8)
Net Income	(6.8)	(29.3)	(26.8)	(37.2)
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Net Loss on Derivatives
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Income Before Income Tax	4.0	8.2	13.8	27.5
Income Tax Expense (Benefit)	0.9	1.7	2.9	5.8
Net Income	(3.1)	(6.5)	(10.9)	(21.7)
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Net Loss on Derivatives | Interest Rate Swaps
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Investment Income	3.4	8.3	13.8	27.6
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Net Loss on Derivatives | Foreign Exchange Contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Investment Loss	0.6	0.0	0.1	0.1
Net Investment Income	0.0	(0.1)	(0.1)	(0.2)
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	12.4	5.4
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	0.0	0.2	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)	(1.8)	(12.2)	(5.3)
Income Tax Expense (Benefit)	(0.9)	(0.4)	(2.6)	(1.1)
Net Income	$ (3.2)	$ (1.4)	$ (9.6)	$ (4.2)