Investment Securities - Schedule of Interest Income on Fixed Maturity Investments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Schedule of Investments [Line Items]
Total investment income	$ 34.7	$ 20.1	$ 81.1	$ 58.8
Third-party investment expenses	(0.8)	(0.6)	(1.5)	(1.6)
Net investment income	33.9	19.5	79.6	57.2
Fixed maturity investments
Schedule of Investments [Line Items]
Total investment income	11.1	8.2	28.6	21.9
Short-term investments
Schedule of Investments [Line Items]
Total investment income	6.7	0.0	7.2	0.5
Common equity securities
Schedule of Investments [Line Items]
Total investment income	0.1	0.1	0.2	0.1
Other long-term investments
Schedule of Investments [Line Items]
Total investment income	17.1	12.0	45.9	37.1
TPC Providers
Schedule of Investments [Line Items]
Total investment income	$ 0.3	$ 0.2	$ 0.8	$ 0.8