Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance			$ 1,175.3
Other comprehensive income (loss) before reclassifications	$ 59.3	$ (53.0)	88.1	$ (26.7)
Amounts reclassified from AOCI	0.4	2.7	2.9	16.2
Other comprehensive income	59.7	(50.3)	91.0	(10.5)
Ending balance	1,287.0	1,051.3	1,287.0	1,051.3
Pretax reclassification amounts from AOCI, tax	(0.1)	(0.7)	(0.8)	(4.3)
Total
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	(282.7)	(359.6)	(314.0)	(399.4)
Ending balance	(223.0)	(409.9)	(223.0)	(409.9)
Net Unrealized Investment Gains (Losses) on Fixed Maturity Securities
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	(370.4)	(393.7)	(328.3)	(449.6)
Other comprehensive income (loss) before reclassifications	132.8	(143.7)	88.2	(101.3)
Amounts reclassified from AOCI	0.4	2.7	2.9	16.2
Other comprehensive income	133.2	(141.0)	91.1	(85.1)
Ending balance	(237.2)	(534.7)	(237.2)	(534.7)
Pretax reclassification amounts from accumulated other comprehensive income (loss)	(0.4)	(3.5)	(3.6)	(20.5)
Net Reserve Remeasurements Attributable to Discount Rates
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	95.3	42.9	21.9	59.0
Other comprehensive income (loss) before reclassifications	(73.5)	90.7	(0.1)	74.6
Amounts reclassified from AOCI	0.0	0.0	0.0	0.0
Other comprehensive income	(73.5)	90.7	(0.1)	74.6
Ending balance	21.8	133.6	21.8	133.6
Net Funded Status of Benefit Plans
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	(7.6)	(8.8)	(7.6)	(8.8)
Other comprehensive income (loss) before reclassifications	0.0	0.0	0.0	0.0
Amounts reclassified from AOCI	0.0	0.0	0.0	0.0
Other comprehensive income	0.0	0.0	0.0	0.0
Ending balance	$ (7.6)	$ (8.8)	$ (7.6)	$ (8.8)

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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