Investments - Net Investment Gains (Losses) by Transaction Type (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Investments, Debt and Equity Securities [Abstract]
Credit loss impairments	$ (0.9)	$ 0.0
Intent-to-sell impairments	0.0	0.0
Total impairments	(0.9)	0.0
Sales and other, net	(0.1)	(2.4)
Change in fair value - equity securities	2.6	(1.0)
Change in fair value and gains (losses) realized on settlements - derivatives	0.6	(0.5)
Net investment gains (losses)	$ 2.2	$ (3.9)