Comprehensive Income (Loss) and Accumulated Other Comprehensive Income (Loss) - Components of Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net income	$ 102.8	$ 45.0	$ 19.8	[1]
Change in net unrealized investment gains (losses) on fixed maturity securities:
Net unrealized investment gains (losses) on securities arising during the period	(58.0)	133.9	(1,042.6)
Less: reclassification adjustment for net investment gains (losses) included in income before income tax	(21.1)	(20.3)	(29.1)
Total, before tax	(36.9)	154.2	(1,013.5)
Income tax expense (benefit)	(7.8)	32.9	(216.8)
Total, net of tax	(29.1)	121.3	(796.7)
Change in net reserve remeasurements attributable to discount rates:
Total, before tax	113.2	(47.2)	567.6
Income tax expense (benefit)	24.2	(10.1)	121.7
Total, net of tax	89.0	(37.1)	445.9	[1]
Change in net funded status of benefit plans:
Total, before tax	0.7	1.5	1.8
Income tax expense (benefit)	0.1	0.3	0.4
Total, net of tax	0.6	1.2	1.4	[1]
Total comprehensive income (loss)	$ 163.3	$ 130.4	$ (329.6)	[1]

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