Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Net Investment Income [Line Items]
Gross investment income	$ 9.0	$ 7.1	$ 24.8	$ 17.9
Investment expense	0.1	0.1	0.2	0.3
Net investment income	8.9	7.0	24.6	17.6
Interest on cash and cash equivalents
Net Investment Income [Line Items]
Net investment income	2.0	0.8	5.8	3.0
Fixed maturities
Net Investment Income [Line Items]
Net investment income	6.5	5.0	17.7	11.9
Short-term investments
Net Investment Income [Line Items]
Net investment income	$ 0.5	$ 1.3	$ 1.3	$ 3.0

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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