Investments - Reconciliation of Net Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	$ 1,175.3
Ending balance	1,211.4	$ 1,139.2
Net unrealized investment gains (losses) on securities
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	(328.3)	(449.6)
Ending balance	(348.0)	(356.4)
Net unrealized investment gains (losses) on securities | Fixed maturity securities
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	(328.3)	(449.6)
Change in net unrealized investment gains (losses) on fixed maturity securities	(20.5)	91.3
Reclassification of net investment losses on fixed maturity securities to net income	0.8	1.9
Ending balance	$ (348.0)	$ (356.4)