Investments - Change in Allowance for Credit Losses on Fixed Maturities Available-for-Sale (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning of the Year	$ 9.6	$ 7.5
Additions for Securities for which No Previous Expected Credit Losses were Recognized	2.9	7.0
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Securities Sold	(2.6)
Net Increase (Decrease) in Allowance on Securities for which Expected Credit Losses were Previously Recognized	(0.9)	5.8
Write-offs Charged Against Allowance	(0.8)	(10.7)
End of the Year	8.2	9.6
Debt Security, Government, Non-US [Member]
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning of the Year	0.0
Additions for Securities for which No Previous Expected Credit Losses were Recognized		(0.7)
Net Increase (Decrease) in Allowance on Securities for which Expected Credit Losses were Previously Recognized		0.0
Write-offs Charged Against Allowance		0.0
End of the Year	0.0	0.0
Corporate Debt Securities [Member]
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning of the Year	8.9	7.5
Additions for Securities for which No Previous Expected Credit Losses were Recognized	2.9	6.3
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Securities Sold	(2.6)
Net Increase (Decrease) in Allowance on Securities for which Expected Credit Losses were Previously Recognized	(1.1)	5.8
Write-offs Charged Against Allowance	(0.4)	(10.7)
End of the Year	7.7	8.9
States and Political Subdivisions
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning of the Year	0.7
Additions for Securities for which No Previous Expected Credit Losses were Recognized	0.0
Debt Securities, Available-for-Sale, Allowance for Credit Loss, Securities Sold	0.0
Net Increase (Decrease) in Allowance on Securities for which Expected Credit Losses were Previously Recognized	(0.2)
Write-offs Charged Against Allowance	(0.4)
End of the Year	$ 0.5	$ 0.7