Investment Securities - Schedule of Pre-Tax Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2025	Mar. 31, 2024
Net Investment Income [Line Items]
Total investment income	$ 60.2	$ 57.7
Third-party investment expenses	(0.6)	(0.7)
Net investment income	59.6	57.0
Fixed maturity investments
Net Investment Income [Line Items]
Total investment income	27.0	20.9
Short-term investments
Net Investment Income [Line Items]
Total investment income	10.6	15.7
Common equity securities
Net Investment Income [Line Items]
Total investment income	0.4	0.5
Other long-term investments
Net Investment Income [Line Items]
Total investment income	$ 22.2	$ 20.6

Amount, before investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

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