Investments - Reconciliation of Net Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance			$ 1,098.3
Ending balance	$ 1,101.9	$ 1,197.2	1,101.9	$ 1,197.2
Net unrealized investment gains (losses) on securities
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	(356.4)	13.0	(449.6)	347.1
Ending balance	(393.7)	(281.8)	(393.7)	(281.8)
Net unrealized investment gains (losses) on securities | Fixed maturity securities
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	(356.4)	13.0	(449.6)	347.1
Change in net unrealized investment gains (losses) on fixed maturity securities	(48.9)	(297.1)	42.4	(633.0)
Reclassification of net investment losses on fixed maturity securities to net income	11.6	2.3	13.5	4.1
Ending balance	$ (393.7)	$ (281.8)	$ (393.7)	$ (281.8)