Comprehensive Income (Loss) and Accumulated Other Comprehensive Income (Loss) - Components of Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Equity [Abstract]
Net income	$ 142.8	$ 133.3	$ 184.4
Change in net unrealized investment gains (losses) on fixed maturity securities:
Net unrealized investment gains (losses) on securities arising during the period	(104.9)	184.0	327.3
Less: reclassification adjustment for net investment gains (losses) included in income before income tax	(8.5)	11.2	157.4
Total, before tax	(96.4)	172.8	169.9
Income tax expense (benefit)	(20.8)	36.9	36.4
Total, net of tax	(75.6)	135.9	133.5
Change in net funded status of benefit plans:
Before tax	1.2	(0.5)	1.8
Income tax expense (benefit)	0.2	(0.1)	0.4
Total, net of tax	1.0	(0.4)	1.4
Comprehensive income	$ 68.2	$ 268.8	$ 319.3