Comprehensive Income (Loss) and Accumulated Other Comprehensive Income (Loss) - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	[1]	$ 1,098.3	$ 1,499.0
Other comprehensive income (loss) before reclassifications		69.4	(372.4)		$ 15.4
Amounts reclassified from AOCI		16.0	23.0		6.1
Net current period other comprehensive income (loss)		85.4	(349.4)	[2]	(405.1)	[2]
Net current period other comprehensive income (loss)					21.5
Ending balance		1,175.3	1,098.3	[1]	1,499.0	[1]
Reclassification adjustment for net investment gains (losses) included in income before income tax		(20.3)	(29.1)		(7.7)
Other tax expense (benefit)		(4.3)	(6.1)		(1.6)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	[1]	(399.4)	(50.0)		355.1
Ending balance		(314.0)	(399.4)	[1]	(50.0)	[1]
Total | Effect of adopting ASU 2018-12
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	[1]				(426.6)
Net unrealized investment gains (losses) on securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		(449.6)	347.1		366.3
Other comprehensive income (loss) before reclassifications		105.3	(819.7)		(96.4)
Amounts reclassified from AOCI		16.0	23.0		6.1
Net current period other comprehensive income (loss)		121.3	(796.7)
Net current period other comprehensive income (loss)					(90.3)
Ending balance		(328.3)	(449.6)		347.1
Net unrealized investment gains (losses) on securities | Effect of adopting ASU 2018-12
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance					71.1
Net Reserve Remeasurements Attributable to Discount Rates
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		59.0	(386.9)		0.0
Other comprehensive income (loss) before reclassifications		(37.1)	445.9		110.8
Amounts reclassified from AOCI		0.0	0.0		0.0
Net current period other comprehensive income (loss)		(37.1)	445.9
Net current period other comprehensive income (loss)					110.8
Ending balance		21.9	59.0		(386.9)
Net Reserve Remeasurements Attributable to Discount Rates | Effect of adopting ASU 2018-12
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance					(497.7)
Net Funded Status of Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		(8.8)	(10.2)		(11.2)
Other comprehensive income (loss) before reclassifications		1.2	1.4		1.0
Amounts reclassified from AOCI		0.0	0.0		0.0
Net current period other comprehensive income (loss)		1.2	1.4
Net current period other comprehensive income (loss)					1.0
Ending balance		$ (7.6)	$ (8.8)		(10.2)
Net Funded Status of Benefit Plans | Effect of adopting ASU 2018-12
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance					$ 0.0

[1] Recast for the adoption of ASU 2018-12. See Note 1 of the Consolidated Financial Statements

[2] Recast for the adoption of ASU 2018-12. See Note 1 of the Consolidated Financial Statements