Investments - Net Investment Gains (Losses) by Transaction Type (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Investments, Debt and Equity Securities [Abstract]
Credit loss impairments	$ 0.9	$ (0.5)	$ 0.0	$ (0.5)
Intent-to-sell impairments	0.0	(6.1)	0.0	(6.1)
Total impairments	0.9	(6.6)	0.0	(6.6)
Sales and other, net	(3.2)	(19.3)	(3.3)	(21.7)
Change in fair value - equity securities	(2.1)	7.7	0.5	6.7
Change in fair value and gains (losses) realized on settlements - derivatives	(1.5)	0.8	(0.9)	0.3
Net investment gains (losses)	$ (5.9)	$ (17.4)	$ (3.7)	$ (21.3)