Investment Securities - Schedule of Interest Income on Fixed Maturity Investments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Schedule of Investments [Line Items]
Total investment income	$ (39.6)	$ (21.0)
Third-party investment expenses	(0.6)	(0.4)
Net investment income	39.0	20.6
Fixed maturity investments
Schedule of Investments [Line Items]
Total investment income	(14.5)	(8.2)
Short-term investments
Schedule of Investments [Line Items]
Total investment income	(9.7)	(0.1)
Common equity securities
Schedule of Investments [Line Items]
Total investment income	(1.3)	0.0
Other long-term investments
Schedule of Investments [Line Items]
Total investment income	(14.1)	(13.2)
TPC Providers
Schedule of Investments [Line Items]
Total investment income	$ 0.0	$ (0.5)