Investments - Allowance for Credit Losses (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Balance, Beginning of Year	$ 10.7	$ 8.2
Additions for Securities for which No Previous Expected Credit Losses were Recognized		0.4
Reductions Due to Sales	(0.5)	(0.7)
Net (Decrease) Increase in Allowance on Securities for which Expected Credit Losses were Previously Recognized	3.5	(0.4)
Write-offs Charged Against Allowance	(1.1)
Balance, End of Period	12.6	7.5
States and Political Subdivisions
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Balance, Beginning of Year	0.3	0.5
Additions for Securities for which No Previous Expected Credit Losses were Recognized		0.0
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)	(0.2)
Write-offs Charged Against Allowance	0.0
Balance, End of Period	0.2	0.3
Corporate Bonds and Notes
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Balance, Beginning of Year	10.4	7.7
Additions for Securities for which No Previous Expected Credit Losses were Recognized		0.4
Reductions Due to Sales	(0.5)	(0.7)
Net (Decrease) Increase in Allowance on Securities for which Expected Credit Losses were Previously Recognized	3.6	(0.2)
Write-offs Charged Against Allowance	(1.1)
Balance, End of Period	$ 12.4	$ 7.2