Investments - Net Unrealized Investment Gains and (Losses) on Fixed Maturities and Equity Securities (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning of period	$ 1,482.7
End of period	1,470.0	$ 1,342.8
Net unrealized investment gains (losses) on securities
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning of period	(245.5)	(357.4)
End of period	(288.8)	(314.5)
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	(245.5)	(357.4)
Change in net unrealized investment gains (losses) on fixed maturity securities	(42.9)	43.0
Reclassification of net investment losses on fixed maturity securities to net income	(0.4)	(0.1)
End of period	$ (288.8)	$ (314.5)