Investments - Net Investment Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Net Investment Income [Line Items]
Gross investment income	$ 34.3	$ 25.0	$ 8.8
Investment expense	0.3	0.3	0.4
Net investment income	34.0	24.7	8.4
Interest on cash and cash equivalents
Net Investment Income [Line Items]
Net investment income	8.3	4.2	1.2
Fixed maturities
Net Investment Income [Line Items]
Net investment income	24.4	17.0	5.7
Short-term investments
Net Investment Income [Line Items]
Net investment income	$ 1.6	$ 3.8	$ 1.9