Investments - Net Investment Gains (Losses) by Transaction Type (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Investments, Debt and Equity Securities [Abstract]
Credit loss impairments	$ 1.0	$ (0.5)	$ (1.2)	$ (0.5)
Intent-to-sell impairments	(0.7)	(0.2)	(1.6)	(0.2)
Total impairments	0.3	(0.7)	(2.8)	(0.7)
Sales and other, net	0.3	0.1	4.4	(3.2)
Change in fair value - equity securities	2.2	5.2	0.3	5.7
Change in fair value and gains (losses) realized on settlements - derivatives	0.5	(0.8)	(7.8)	(1.7)
Net investment gains (losses)	$ 3.3	$ 3.8	$ (5.9)	$ 0.1