Investment Securities - Schedule of Pre-Tax Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Net Investment Income [Line Items]
Total investment income	$ 58.4	$ 48.8	$ 174.3	$ 131.8
Third-party investment expenses	(0.7)	(0.7)	(2.2)	(1.7)
Net investment income	57.7	48.1	172.1	130.1
Fixed maturity investments
Net Investment Income [Line Items]
Total investment income	23.3	16.6	67.2	46.6
Short-term investments
Net Investment Income [Line Items]
Total investment income	13.2	13.8	45.3	34.8
Common equity securities
Net Investment Income [Line Items]
Total investment income	0.7	1.0	1.8	3.6
Other long-term investments
Net Investment Income [Line Items]
Total investment income	$ 21.2	$ 17.4	$ 60.0	$ 46.8