Investments - Reconciliation of Net Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	$ 1,098.3
Ending balance	1,139.2	$ 1,351.5
Net unrealized investment gains (losses) on securities
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	(449.6)	347.1
Ending balance	(356.4)	13.0
Net unrealized investment gains (losses) on securities | Fixed maturity securities
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	(449.6)	347.1
Change in net unrealized investment gains (losses) on fixed maturity securities	91.3	(335.9)
Reclassification of net investment losses on fixed maturity securities to net income	1.9	1.8
Ending balance	$ (356.4)	$ 13.0