Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Net Investment Income [Line Items]
Total	$ 9.5	$ 8.2	$ 19.1	$ 15.9
Investment expense	0.1	0.1	0.2	0.2
Net investment income	9.4	8.1	18.9	15.7
Interest on cash and cash equivalents
Net Investment Income [Line Items]
Net investment income	2.1	2.2	4.2	3.7
Fixed maturities
Net Investment Income [Line Items]
Net investment income	7.2	5.6	14.4	11.4
Short-term investments
Net Investment Income [Line Items]
Net investment income	$ 0.2	$ 0.4	$ 0.5	$ 0.8