Investment Securities - Schedule of Pre-Tax Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Net Investment Income [Line Items]
Total investment income	$ 58.2	$ 43.4	$ 115.9	$ 83.0
Third-party investment expenses	(0.8)	(0.4)	(1.5)	(1.0)
Net investment income	57.4	43.0	114.4	82.0
Fixed maturity investments
Net Investment Income [Line Items]
Total investment income	23.2	15.5	43.9	30.0
Short-term investments
Net Investment Income [Line Items]
Total investment income	16.0	11.3	32.0	21.0
Common equity securities
Net Investment Income [Line Items]
Total investment income	0.6	1.3	1.1	2.6
Other long-term investments
Net Investment Income [Line Items]
Total investment income	$ 18.4	$ 15.3	$ 38.9	$ 29.4