Investments - Net Investment Gains (Losses) by Transaction Type (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Investments, Debt and Equity Securities [Abstract]
Credit loss impairments	$ (0.5)	$ 0.0	$ (0.5)	$ (0.5)
Intent-to-sell impairments	(0.2)	(0.6)	(0.2)	(6.7)
Total impairments	(0.7)	(0.6)	(0.7)	(7.2)
Sales and other, net	0.1	(3.5)	(3.2)	(25.2)
Change in fair value - equity securities	5.2	(1.4)	5.7	5.3
Change in fair value and gains (losses) realized on settlements - derivatives	(0.8)	(2.9)	(1.7)	(2.6)
Net investment gains (losses)	$ 3.8	$ (8.4)	$ 0.1	$ (29.7)