Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning of period	$ 1,482.7
Other comprehensive income (loss) before reclassifications	(20.2)	$ 31.2
Amounts reclassified from AOCI	(0.4)	(0.1)
Net current period other comprehensive income (loss)	(20.6)	31.1
End of period	1,470.0	1,342.8
Pretax amounts reclassified from AOCI	0.5	0.2
Income tax expense (benefit) amounts reclassified from AOCI	0.1	0.1
Total
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning of period	(154.6)	(253.5)
End of period	(175.2)	(222.4)
Net Unrealized Investment Gains (Losses) on Fixed Maturity Securities
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning of period	(245.5)	(357.4)
Other comprehensive income (loss) before reclassifications	(42.9)	43.0
Amounts reclassified from AOCI	(0.4)	(0.1)
Net current period other comprehensive income (loss)	(43.3)	42.9
End of period	(288.8)	(314.5)
Net Reserve Remeasurements Attributable to Discount Rates
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning of period	93.4	110.9
Other comprehensive income (loss) before reclassifications	22.7	(11.8)
Amounts reclassified from AOCI	0.0	0.0
Net current period other comprehensive income (loss)	22.7	(11.8)
End of period	116.1	99.1
Net Funded Status of Benefit Plans
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning of period	(2.5)	(7.0)
Other comprehensive income (loss) before reclassifications	0.0	0.0
Amounts reclassified from AOCI	0.0	0.0
Net current period other comprehensive income (loss)	0.0	0.0
End of period	$ (2.5)	$ (7.0)