Schedule of Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Investment Loss		$ (34.6)	$ (36.0)	$ (15.7)
Impairement Loss	[1]	(2.5)	0.0	0.0
Change in Allowance for Credit Losses		3.0	2.2	4.6
Net Investment Income		2,130.0	2,096.7	2,122.2
Other Expenses		(1,222.7)	(1,112.0)	(1,006.7)
Income Tax Expense (Benefit)		472.2	356.3	342.8
Net Income		1,779.1	1,283.8	1,407.2
Reclassification out of Accumulated Other Comprehensive Income | Net Unrealized Gain (Loss) on Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Investment Loss		(38.2)	(48.7)	(24.8)
Impairement Loss		(2.5)	0.0	0.0
Change in Allowance for Credit Losses		(3.0)	(2.2)	(4.6)
Income of Parent Company Before Income Tax		(43.7)	(50.9)	(29.4)
Income Tax Expense (Benefit)		(9.4)	(10.9)	(6.2)
Net Income		(34.3)	(40.0)	(23.2)
Reclassification out of Accumulated Other Comprehensive Income | Net Gain (Loss) on Derivatives
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Income of Parent Company Before Income Tax		16.0	33.8	51.9
Income Tax Expense (Benefit)		3.4	7.1	10.9
Net Income		12.6	26.7	41.0
Reclassification out of Accumulated Other Comprehensive Income | Net Gain (Loss) on Derivatives | Interest Rate Swaps
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Investment Income		16.4	34.7	51.1
Reclassification out of Accumulated Other Comprehensive Income | Net Gain (Loss) on Derivatives | Foreign Exchange Contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Investment Loss		(0.2)	(0.4)	1.8
Net Investment Income		(0.2)	(0.5)	(1.0)
Reclassification out of Accumulated Other Comprehensive Income | Net Actuarial Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses		(16.4)	(7.2)	(15.7)
Reclassification out of Accumulated Other Comprehensive Income | Prior Service Credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses		0.1	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 of Parent Company Before Income Tax		(16.3)	(7.0)	(15.5)
Income Tax Expense (Benefit)		(3.4)	(1.7)	(3.3)
Net Income		$ (12.9)	$ (5.3)	$ (12.2)

[1] 1Includes write-down of securities that we intended to sell prior to recovery of the amortized cost basis.