Comprehensive Income (Loss) and Accumulated Other Comprehensive Income (Loss) - Components of Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Equity [Abstract]
Net income	$ 162.1	$ 102.8	$ 45.0
Change in net unrealized investment gains (losses) on fixed maturity securities:
Net unrealized investment gains (losses) on securities arising during the period	135.9	(58.0)	133.9
Less: reclassification adjustment for net investment gains (losses) included in income before income tax	(6.5)	(21.1)	(20.3)
Total, before tax	142.4	(36.9)	154.2
Income tax expense (benefit)	30.5	(7.8)	32.9
Total, net of tax	111.9	(29.1)	121.3
Change in net reserve remeasurements attributable to discount rates:
Total, before tax	(22.2)	113.2	(47.2)
Income tax expense (benefit)	(4.7)	24.2	(10.1)
Total, net of tax	(17.5)	89.0	(37.1)
Change in net funded status of benefit plans:
Total, before tax	5.7	0.7	1.5
Income tax expense (benefit)	1.2	0.1	0.3
Total, net of tax	4.5	0.6	1.2
Comprehensive income	$ 261.0	$ 163.3	$ 130.4