Investments - Net Unrealized Investment Gains and Losses on Fixed Maturities and Equity Securities (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	[1]	$ 1,098.3	$ 1,499.0
Ending balance		1,175.3	1,098.3	[1]	$ 1,499.0	[1]
Net unrealized investment gains (losses) on securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		(449.6)	347.1		366.3
Ending balance		(328.3)	(449.6)		347.1
Net unrealized investment gains (losses) on securities | Effect of adopting ASU 2018-12
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance					71.1
Net unrealized investment gains (losses) on securities | Fixed maturity securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		(449.6)	347.1		366.3
Change in net unrealized investment gains (losses) on fixed maturity securities		105.3	(819.7)		(96.4)
Reclassification of net investment (gains) losses on fixed maturity securities to net income		16.0	23.0		6.1
Ending balance		$ (328.3)	$ (449.6)		347.1
Net unrealized investment gains (losses) on securities | Fixed maturity securities | Effect of adopting ASU 2018-12
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance					$ 71.1

[1] Recast for the adoption of ASU 2018-12. See Note 1 of the Consolidated Financial Statements