Schedule of Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2026	Mar. 31, 2025
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Investment Loss		$ (5.0)	$ (206.8)
Impairment Loss, Debt Securities, Available-for-sale	[1]	(8.1)	(152.4)
Credit Losses		(5.9)	1.0
Net Investment Income		483.4	513.2
Other Expenses		(357.8)	(318.6)
Income Tax Expense (Benefit)		70.7	54.5
Net Income		232.0	189.1
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Net Unrealized Loss on Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Investment Loss		(0.5)	(44.7)
Impairment Loss, Debt Securities, Available-for-sale		(8.1)	(152.4)
Credit Losses		(5.9)	1.0
Income Before Income Tax		(2.7)	(198.1)
Income Tax Expense (Benefit)		(0.6)	(41.7)
Net Income		(2.1)	(156.4)
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Net Loss on Derivatives
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Income Before Income Tax		1.7	2.5
Income Tax Expense (Benefit)		0.4	0.5
Net Income		1.3	2.0
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 Loss		0.0	0.4
Net Investment Income		0.8	2.2
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		0.9	(0.1)
Reclassification out of Accumulated Other Comprehensive Income (Loss) | Amortization of Net Actuarial Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses		2.7	4.1
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		(2.6)	(4.0)
Income Tax Expense (Benefit)		(0.5)	(0.8)
Net Income		$ (2.1)	$ (3.2)

[1] 2 During the three months ended March 31, 2025, we recognized a $152.4 million impairment loss based on the intent to dispose of fixed maturity securities with a fair value of $1,250.9 million related to the Fortitude Re reinsurance transaction.