Investments - Reconciliation of Net Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning of period			$ 1,287.5
End of period	$ 1,441.3	$ 1,287.0	1,441.3	$ 1,287.0
Net unrealized investment gains (losses) on securities
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning of period	(309.9)	(370.4)	(357.4)	(328.3)
End of period	(250.8)	(237.2)	(250.8)	(237.2)
Net unrealized investment gains (losses) on securities | Fixed maturity securities
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning of period	(309.9)	(370.4)	(357.4)	(328.3)
Change in net unrealized investment gains (losses) on fixed maturity securities	58.7	132.8	103.1	88.2
Reclassification of net investment losses on fixed maturity securities to net income	0.4	0.4	3.5	2.9
End of period	$ (250.8)	$ (237.2)	$ (250.8)	$ (237.2)