Comprehensive Income (Loss) and Accumulated Other Comprehensive Income (Loss) - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023		Dec. 31, 2022
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 1,175.3	$ 1,098.3	[1]
Other comprehensive income (loss) before reclassifications	43.8	69.4		$ (372.4)
Amounts reclassified from AOCI	16.7	16.0		23.0
Net current period other comprehensive income (loss)	60.5	85.4		(349.4)	[2]
Net current period other comprehensive income (loss)				(349.4)
Ending balance	1,287.5	1,175.3		1,098.3	[1]
Reclassification adjustment for net investment gains (losses) included in income before income tax	(21.1)	(20.3)		(29.1)
Other tax expense (benefit)	(4.4)	(4.3)		(6.1)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(314.0)	(399.4)	[1]	(50.0)	[1]
Ending balance	(253.5)	(314.0)		(399.4)	[1]
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(328.3)	(449.6)		347.1
Other comprehensive income (loss) before reclassifications	(45.8)	105.3		(819.7)
Amounts reclassified from AOCI	16.7	16.0		23.0
Net current period other comprehensive income (loss)	(29.1)	121.3
Net current period other comprehensive income (loss)				(796.7)
Ending balance	(357.4)	(328.3)		(449.6)
Net Reserve Remeasurements Attributable to Discount Rates
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	21.9	59.0		(386.9)
Other comprehensive income (loss) before reclassifications	89.0	(37.1)		445.9
Amounts reclassified from AOCI	0.0	0.0		0.0
Net current period other comprehensive income (loss)	89.0	(37.1)
Net current period other comprehensive income (loss)				445.9
Ending balance	110.9	21.9		59.0
Net Funded Status of Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(7.6)	(8.8)		(10.2)
Other comprehensive income (loss) before reclassifications	0.6	1.2		1.4
Amounts reclassified from AOCI	0.0	0.0		0.0
Net current period other comprehensive income (loss)	0.6	1.2
Net current period other comprehensive income (loss)				1.4
Ending balance	$ (7.0)	$ (7.6)		$ (8.8)

[1] Recast for the adoption of ASU 2018-12.

[2] Recast for the adoption of ASU 2018-12.