Comprehensive Income (Loss) and Accumulated Other Comprehensive Income (Loss) - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 1,287.5	$ 1,175.3
Other comprehensive income (loss) before reclassifications	93.7	43.8	$ 69.4
Amounts reclassified from AOCI	5.2	16.7	16.0
Net current period other comprehensive income (loss)	98.9	60.5	85.4
Net current period other comprehensive income (loss)			85.4
Ending balance	1,482.7	1,287.5	1,175.3
Pretax amounts reclassified from AOCI	(6.6)	(21.1)	(20.3)
Income tax expense (benefit) amounts reclassified from AOCI	(1.4)	(4.4)	(4.3)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(253.5)	(314.0)	(399.4)
Ending balance	(154.6)	(253.5)	(314.0)
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(357.4)	(328.3)	(449.6)
Other comprehensive income (loss) before reclassifications	106.7	(45.8)	105.3
Amounts reclassified from AOCI	5.2	16.7	16.0
Net current period other comprehensive income (loss)	111.9	(29.1)
Net current period other comprehensive income (loss)			121.3
Ending balance	(245.5)	(357.4)	(328.3)
Net Reserve Remeasurements Attributable to Discount Rates
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	110.9	21.9	59.0
Other comprehensive income (loss) before reclassifications	(17.5)	89.0	(37.1)
Amounts reclassified from AOCI	0.0	0.0	0.0
Net current period other comprehensive income (loss)	(17.5)	89.0
Net current period other comprehensive income (loss)			(37.1)
Ending balance	93.4	110.9	21.9
Net Funded Status of Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(7.0)	(7.6)	(8.8)
Other comprehensive income (loss) before reclassifications	4.5	0.6	1.2
Amounts reclassified from AOCI	0.0	0.0	0.0
Net current period other comprehensive income (loss)	4.5	0.6
Net current period other comprehensive income (loss)			1.2
Ending balance	$ (2.5)	$ (7.0)	$ (7.6)