Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning of period	$ 1,287.5
Other comprehensive income (loss) before reclassifications	31.2	$ 20.9
Amounts reclassified from AOCI	(0.1)	0.8
Other comprehensive income	31.1	21.7
End of period	1,342.8	1,211.4
Pretax reclassification amounts from accumulated other comprehensive income (loss)	0.2	(1.0)
Pretax reclassification amounts from AOCI, tax	0.1	(0.2)
Total
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning of period	(253.5)	(314.0)
End of period	(222.4)	(292.3)
Net Unrealized Investment Gains (Losses) on Fixed Maturity Securities
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning of period	(357.4)	(328.3)
Other comprehensive income (loss) before reclassifications	43.0	(20.5)
Amounts reclassified from AOCI	(0.1)	0.8
Other comprehensive income	42.9	(19.7)
End of period	(314.5)	(348.0)
Net Reserve Remeasurements Attributable to Discount Rates
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning of period	110.9	21.9
Other comprehensive income (loss) before reclassifications	(11.8)	41.4
Amounts reclassified from AOCI	0.0	0.0
Other comprehensive income	(11.8)	41.4
End of period	99.1	63.3
Net Funded Status of Benefit Plans
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning of period	(7.0)	(7.6)
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
End of period	$ (7.0)	$ (7.6)