Investment Securities - Schedule of Interest Income on Fixed Maturity Investments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Schedule of Investments [Line Items]
Total investment income	$ 25.5	$ 17.7	$ 46.5	$ 38.7
Third-party investment expenses	(0.4)	(0.6)	(0.8)	(1.0)
Net investment income	25.1	17.1	45.7	37.7
Fixed maturity investments
Schedule of Investments [Line Items]
Total investment income	9.4	7.2	17.6	13.7
Short-term investments
Schedule of Investments [Line Items]
Total investment income	0.4	(1.1)	0.5	0.5
Common equity securities
Schedule of Investments [Line Items]
Total investment income	0.1	0.0	0.1	0.0
Other long-term investments
Schedule of Investments [Line Items]
Total investment income	15.6	11.3	28.8	25.1
TPC Providers
Schedule of Investments [Line Items]
Total investment income	$ 0.0	$ 0.3	$ 0.5	$ 0.6