Investment Securities - Schedule of Pre-Tax Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Net Investment Income [Line Items]
Total investment income	$ 48.8	$ 34.7	$ 131.8	$ 81.1
Third-party investment expenses	(0.7)	(0.8)	(1.7)	(1.5)
Net investment income	48.1	33.9	130.1	79.6
Fixed maturity investments
Net Investment Income [Line Items]
Total investment income	16.6	11.1	46.6	28.6
Short-term investments
Net Investment Income [Line Items]
Total investment income	13.8	6.7	34.8	7.2
Common equity securities
Net Investment Income [Line Items]
Total investment income	1.0	0.1	3.6	0.2
Other long-term investments
Net Investment Income [Line Items]
Total investment income	17.4	17.1	46.8	45.9
Amount attributable to TPC Providers
Net Investment Income [Line Items]
Total investment income	$ 0.0	$ 0.3	$ 0.0	$ 0.8