Investment Securities - Schedule of Pre-Tax Net Investment Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Net Investment Income [Line Items]
Total investment income	$ 250.6	$ 230.2	$ 196.6
Third-party investment expenses	(2.6)	(2.8)	(2.4)
Net investment income, pre-tax	248.0	227.4	194.2
Fixed maturity investments
Net Investment Income [Line Items]
Total investment income	111.8	93.2	64.4
Short-term investments
Net Investment Income [Line Items]
Total investment income	47.8	56.6	52.0
Common equity securities
Net Investment Income [Line Items]
Total investment income	0.6	2.5	4.1
Other long-term investments
Net Investment Income [Line Items]
Total investment income	$ 90.4	$ 77.9	$ 76.1