Investments - Reconciliation of Net Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance			$ 1,175.3
Ending balance	$ 1,287.0	$ 1,051.3	1,287.0	$ 1,051.3
Net unrealized investment gains (losses) on securities
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	(370.4)	(393.7)	(328.3)	(449.6)
Ending balance	(237.2)	(534.7)	(237.2)	(534.7)
Net unrealized investment gains (losses) on securities | Fixed maturity securities
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	(370.4)	(393.7)	(328.3)	(449.6)
Change in net unrealized investment gains (losses) on fixed maturity securities	132.8	(143.7)	88.2	(101.3)
Reclassification of net investment losses on fixed maturity securities to net income	0.4	2.7	2.9	16.2
Ending balance	$ (237.2)	$ (534.7)	$ (237.2)	$ (534.7)