Investment Securities - Schedule of Pre-Tax Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Net Investment Income [Line Items]
Total investment income	$ 59.8	$ 58.4	$ 182.1	$ 174.3
Third-party investment expenses	(0.6)	(0.7)	(1.9)	(2.2)
Net investment income	59.2	57.7	180.2	172.1
Fixed maturity investments
Net Investment Income [Line Items]
Total investment income	28.9	23.3	82.7	67.2
Short-term investments
Net Investment Income [Line Items]
Total investment income	11.8	13.2	34.4	45.3
Common equity securities
Net Investment Income [Line Items]
Total investment income	0.0	0.7	0.6	1.8
Other long-term investments
Net Investment Income [Line Items]
Total investment income	$ 19.1	$ 21.2	$ 64.4	$ 60.0