Investments - Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Net Investment Income [Line Items]
Gross investment income	$ 5.7	$ 1.3	$ 10.8	$ 2.3
Investment expense	0.1	0.1	0.2	0.2
Net investment income	5.6	1.2	10.6	2.1
Interest on cash and cash equivalents
Net Investment Income [Line Items]
Net investment income	1.0	0.1	2.2	0.1
Fixed maturities
Net Investment Income [Line Items]
Net investment income	4.0	1.1	6.9	2.0
Short-term investments
Net Investment Income [Line Items]
Net investment income	$ 0.7	$ 0.1	$ 1.7	$ 0.2