Investments - Net Unrealized Investment Gains and Losses on Fixed Maturities and Equity Securities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023		Dec. 31, 2022
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 1,175.3	$ 1,098.3	[1]
Ending balance	1,287.5	1,175.3		$ 1,098.3	[1]
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(328.3)	(449.6)		347.1
Ending balance	(357.4)	(328.3)		(449.6)
Net unrealized investment gains (losses) on fixed maturity securities, net of tax | Fixed maturity securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(328.3)	(449.6)		347.1
Change in net unrealized investment gains (losses) on fixed maturity securities	(45.8)	105.3		(819.7)
Reclassification of net investment (gains) losses on fixed maturity securities to net income	16.7	16.0		23.0
Ending balance	$ (357.4)	$ (328.3)		$ (449.6)

[1] Recast for the adoption of ASU 2018-12.