Investment Securities - Schedule of Pre-Tax Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Net Investment Income [Line Items]
Total investment income	$ 62.1	$ 58.2	$ 122.3	$ 115.9
Third-party investment expenses	(0.7)	(0.8)	(1.3)	(1.5)
Net investment income	61.4	57.4	121.0	114.4
Fixed maturity investments
Net Investment Income [Line Items]
Total investment income	26.8	23.2	53.8	43.9
Short-term investments
Net Investment Income [Line Items]
Total investment income	12.0	16.0	22.6	32.0
Common equity securities
Net Investment Income [Line Items]
Total investment income	0.2	0.6	0.6	1.1
Other long-term investments
Net Investment Income [Line Items]
Total investment income	$ 23.1	$ 18.4	$ 45.3	$ 38.9

Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.