Investment Securities - Schedule of Pre-Tax Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Net Investment Income [Line Items]
Total investment income	$ 57.7	$ 39.6
Third-party investment expenses	(0.7)	(0.6)
Net investment income	57.0	39.0
Fixed maturity investments
Net Investment Income [Line Items]
Total investment income	20.9	14.5
Short-term investments
Net Investment Income [Line Items]
Total investment income	15.7	9.7
Common equity securities
Net Investment Income [Line Items]
Total investment income	0.5	1.3
Other long-term investments
Net Investment Income [Line Items]
Total investment income	$ 20.6	$ 14.1