Investment Securities - Schedule of Pre-Tax Net Investment Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Net Investment Income [Line Items]
Total investment income	$ 230.2	$ 196.6	$ 126.5
Third-party investment expenses	(2.8)	(2.4)	(2.1)
Net investment income, pre-tax	227.4	194.2	124.4
Fixed maturity investments
Net Investment Income [Line Items]
Total investment income	93.2	64.4	41.4
Short-term investments
Net Investment Income [Line Items]
Total investment income	56.6	52.0	16.4
Common equity securities
Net Investment Income [Line Items]
Total investment income	2.5	4.1	1.6
Other long-term investments
Net Investment Income [Line Items]
Total investment income	77.9	76.1	68.1
Amount attributable to TPC Providers
Net Investment Income [Line Items]
Amount attributable to TPC Providers	$ 0.0	$ 0.0	$ (1.0)