Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Net Investment Income [Line Items]
Total	$ 9.8	$ 9.0	$ 28.9	$ 24.8
Investment expense	0.1	0.1	0.3	0.2
Net investment income	9.7	8.9	28.6	24.6
Realized investment	0.2	0.1	0.3	0.1
Interest on cash and cash equivalents
Net Investment Income [Line Items]
Net investment income	2.1	2.0	6.3	5.8
Fixed maturities
Net Investment Income [Line Items]
Net investment income	7.5	6.5	21.9	17.7
Short-term investments
Net Investment Income [Line Items]
Net investment income	$ 0.2	$ 0.5	$ 0.7	$ 1.3