Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Net Investment Income [Line Items]
Gross investment income	$ 7.1	$ 2.7	$ 17.9	$ 5.0
Investment expense	0.1	0.1	0.3	0.3
Net investment income	7.0	2.6	17.6	4.7
Interest on cash and cash equivalents
Net Investment Income [Line Items]
Net investment income	0.8	0.3	3.0	0.4
Fixed maturities
Net Investment Income [Line Items]
Net investment income	5.0	1.7	11.9	3.7
Short-term investments
Net Investment Income [Line Items]
Net investment income	$ 1.3	$ 0.7	$ 3.0	$ 0.9