Investments - Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Balance, Beginning of Year	$ 10.7	$ 8.2
Reductions Due to Sales	(0.5)	(0.2)
Net (Decrease) Increase in Allowance on Securities for which Expected Credit Losses were Previously Recognized	0.2	(0.3)
Write-offs Charged Against Allowance	(1.1)
Balance, End of Period	9.3	7.7
States and Political Subdivisions
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Balance, Beginning of Year	0.3	0.5
Reductions Due to Sales	0.0	0.0
Net (Decrease) Increase in Allowance on Securities for which Expected Credit Losses were Previously Recognized	(0.1)	0.0
Write-offs Charged Against Allowance	0.0
Balance, End of Period	0.2	0.5
Corporate Bonds and Notes
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Balance, Beginning of Year	10.4	7.7
Reductions Due to Sales	(0.5)	(0.2)
Net (Decrease) Increase in Allowance on Securities for which Expected Credit Losses were Previously Recognized	0.3	(0.3)
Write-offs Charged Against Allowance	(1.1)
Balance, End of Period	$ 9.1	$ 7.2