Investments - Net Investment Gains (Losses) By Transaction Type (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021
Investments, Debt and Equity Securities [Abstract]
Credit loss impairments	$ (0.4)	$ (3.1)		$ (8.1)
Intent-to-sell impairments	(6.7)	(7.6)		(2.3)
Total impairments	(7.1)	(10.7)		(10.4)
Sales and other, net	(25.0)	(17.8)		4.3
Change in fair value - equity securities	7.9	(33.2)		(2.3)
Change in fair value and losses realized on settlements - derivatives	0.2	5.2		(2.6)
Net investment losses	$ (24.0)	$ (56.5)	[1]	$ (11.0)	[1]

[1] Recast for the adoption of ASU 2018-12. See Note 1 of the Consolidated Financial Statements