Schedule of Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Investment Gain (Loss)	$ 0.9	$ (4.1)	$ 1.0	$ (17.9)
Credit Losses on Fixed Matruity Securities	0.0	0.0	0.0	(4.1)
Net Investment Income	531.1	559.0	1,039.9	1,086.2
Other Expenses	(273.6)	(250.0)	(546.8)	(490.5)
Income Tax Expense (Benefit)	105.5	71.9	198.5	127.2
Net Income	392.9	367.3	751.2	607.7
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)	(10.3)	(3.7)	(10.0)	(12.2)
Credit Losses on Fixed Matruity Securities	0.0	0.0	0.0	(4.1)
Income Before Income Tax	(10.3)	(3.7)	(10.0)	(16.3)
Income Tax Expense (Benefit)	(2.2)	(0.9)	(2.1)	(3.5)
Net Income	(8.1)	(2.8)	(7.9)	(12.8)
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Net Gain (Loss) on Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Income Before Income Tax	8.9	13.2	19.3	26.9
Income Tax Expense (Benefit)	1.9	2.7	4.1	5.6
Net Income	(7.0)	(10.5)	(15.2)	(21.3)
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Net Gain (Loss) on Hedges | Interest Rate Swaps
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Investment Income	8.9	13.0	19.3	26.4
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Net Gain (Loss) on Hedges | Foreign Exchange Contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Investment Gain (Loss)	0.1	0.0	0.1	0.0
Net Investment Income	(0.1)	0.2	(0.1)	0.5
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)	(3.8)	(3.6)	(7.8)
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Prior Service Credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses	0.0	0.0	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	(1.8)	(3.8)	(3.5)	(7.7)
Income Tax Expense (Benefit)	(0.3)	(0.8)	(0.7)	(1.6)
Net Income	$ (1.5)	$ (3.0)	$ (2.8)	$ (6.1)