Investment Securities - Schedule of Interest Income on Fixed Maturity Investments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Schedule of Investments [Line Items]
Total investment income	$ 43.4	$ 25.5	$ 83.0	$ 46.5
Third-party investment expenses	(0.4)	(0.4)	(1.0)	(0.8)
Net investment income	43.0	25.1	82.0	45.7
Fixed maturity investments
Schedule of Investments [Line Items]
Total investment income	15.5	9.4	30.0	17.6
Short-term investments
Schedule of Investments [Line Items]
Total investment income	11.3	0.4	21.0	0.5
Common equity securities
Schedule of Investments [Line Items]
Total investment income	1.3	0.1	2.6	0.1
Other long-term investments
Schedule of Investments [Line Items]
Total investment income	15.3	15.6	29.4	28.8
TPC Providers
Schedule of Investments [Line Items]
Total investment income	$ 0.0	$ 0.0	$ 0.0	$ 0.5