Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Net Investment Income [Line Items]
Gross investment income	$ 2.7	$ 0.7	$ 5.0	$ 1.1
Investment expense	0.1	0.1	0.3	0.1
Net investment income	2.6	0.6	4.7	1.0
Interest on cash and cash equivalents
Net Investment Income [Line Items]
Net investment income	0.3	0.1	0.4	0.4
Fixed maturities
Net Investment Income [Line Items]
Net investment income	1.7	0.6	3.7	0.7
Short-term investments
Net Investment Income [Line Items]
Net investment income	$ 0.7	$ 0.0	$ 0.9	$ 0.0