Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	$ 1,175.3
Other comprehensive income (loss) before reclassifications	20.9	$ 50.1
Amounts reclassified from AOCI	0.8	1.9
Other comprehensive income	21.7	52.0
Ending balance	1,211.4	1,139.2
Pretax reclassification amounts from AOCI, tax	(0.2)	(0.5)
Total
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	(314.0)	(399.4)
Ending balance	(292.3)	(347.4)
Net Unrealized Investment Gains (Losses) on Fixed Maturity Securities
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	(328.3)	(449.6)
Other comprehensive income (loss) before reclassifications	(20.5)	91.3
Amounts reclassified from AOCI	0.8	1.9
Other comprehensive income	(19.7)	93.2
Ending balance	(348.0)	(356.4)
Pretax reclassification amounts from accumulated other comprehensive income (loss)	(1.0)	(2.4)
Net Reserve Remeasurements Attributable to Discount Rates
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	21.9	59.0
Other comprehensive income (loss) before reclassifications	41.4	(41.2)
Amounts reclassified from AOCI	0.0	0.0
Other comprehensive income	41.4	(41.2)
Ending balance	63.3	17.8
Net Funded Status of Benefit Plans
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	(7.6)	(8.8)
Other comprehensive income (loss) before reclassifications	0.0	0.0
Amounts reclassified from AOCI	0.0	0.0
Other comprehensive income	0.0	0.0
Ending balance	$ (7.6)	$ (8.8)