Investments - Schedule of Changes in Unrealized Appreciation (Depreciation) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2025	Mar. 31, 2024
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	$ 9.2	$ (20.3)
Income tax provision (benefit)	2.0	(4.4)
Net unrealized investment gains (losses), net of income taxes	7.2	(15.9)
Fixed maturities
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	9.4	(19.9)
Other and short-term investments
Schedule of Investments [Line Items]
Net unrealized investment gains (losses) before income taxes	$ (0.2)	$ (0.4)

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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