Investments - Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Balance, Beginning of Year	$ (8.2)	$ 9.6
Additions for Securities for which No Previous Expected Credit Losses were Recognized		(0.3)
Reductions Due to Sales	(0.2)	(1.9)
Net Decrease in Allowance on Securities for which Expected Credit Losses were Previously Recognized	(0.3)	0.6
Debt Securities, Available-for-Sale, Allowance for Credit Loss	(7.7)	7.4
Equity securities, FV-NI, unrealized gain (loss)	3.9	4.4
Foreign Governments
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Balance, Beginning of Year	0.0	0.7
Additions for Securities for which No Previous Expected Credit Losses were Recognized		0.0
Net Decrease in Allowance on Securities for which Expected Credit Losses were Previously Recognized		(0.1)
Debt Securities, Available-for-Sale, Allowance for Credit Loss	0.0	0.8
Corporate Debt Securities [Member]
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Balance, Beginning of Year	(7.7)	8.9
Additions for Securities for which No Previous Expected Credit Losses were Recognized		(0.3)
Reductions Due to Sales	(0.2)	(1.9)
Net Decrease in Allowance on Securities for which Expected Credit Losses were Previously Recognized	(0.3)	0.7
Debt Securities, Available-for-Sale, Allowance for Credit Loss	(7.2)	6.6
States and Political Subdivisions
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Balance, Beginning of Year	(0.5)
Reductions Due to Sales	0.0	$ 0.0
Net Decrease in Allowance on Securities for which Expected Credit Losses were Previously Recognized	0.0
Debt Securities, Available-for-Sale, Allowance for Credit Loss	$ (0.5)