Investments - Change in Allowance for Credit Losses on Fixed Maturities Available-for-Sale (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning of the Year	$ 7.5	$ 3.3
Additions for Securities for which No Previous Expected Credit Losses were Recognized	7.0	5.6
Net Increase in Allowance on Securities for which Expected Credit Losses were Previously Recognized	5.8	(1.1)
Write-offs Charged Against Allowance	(10.7)	(0.3)
End of Year	9.6	7.5
Foreign Governments
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning of the Year	0.0	0.3
Additions for Securities for which No Previous Expected Credit Losses were Recognized	(0.7)	0.0
Net Increase in Allowance on Securities for which Expected Credit Losses were Previously Recognized	0.0	(0.3)
Write-offs Charged Against Allowance	0.0
End of Year	0.0	0.0
Corporate Bonds and Notes
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning of the Year	7.5	3.0
Additions for Securities for which No Previous Expected Credit Losses were Recognized	6.3	5.6
Net Increase in Allowance on Securities for which Expected Credit Losses were Previously Recognized	5.8	(0.8)
Write-offs Charged Against Allowance	(10.7)	(0.3)
End of Year	8.9	7.5
States and Political Subdivisions
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning of the Year	0.0
End of Year	$ 0.7	$ 0.0