Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance			$ 1,098.3
Other comprehensive income (loss) before reclassifications	$ (23.8)	$ (128.6)	26.3	$ (283.0)
Amounts reclassified from AOCI	11.6	2.3	13.5	4.1
Other comprehensive loss	(12.2)	(126.3)	39.8	(278.9)
Ending balance	1,101.9	1,197.2	1,101.9	1,197.2
Pretax reclassification amounts from AOCI, tax	(3.1)	(0.6)	(3.6)	(1.1)
AOCI
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	(347.4)	(202.6)	(399.4)	(50.0)
Ending balance	(359.6)	(328.9)	(359.6)	(328.9)
Net Unrealized Investment Gains (Losses) on Fixed Maturity Securities
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	(356.4)	13.0	(449.6)	347.1
Other comprehensive income (loss) before reclassifications	(48.9)	(297.1)	42.4	(633.0)
Amounts reclassified from AOCI	11.6	2.3	13.5	4.1
Other comprehensive loss	(37.3)	(294.8)	55.9	(628.9)
Ending balance	(393.7)	(281.8)	(393.7)	(281.8)
Pretax reclassification amounts from accumulated other comprehensive income (loss)	(14.6)	(2.9)	(17.0)	(5.2)
Net Reserve Remeasurements Attributable to Discount Rates
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	17.8	(205.4)	59.0	(386.9)
Other comprehensive income (loss) before reclassifications	25.1	168.5	(16.1)	350.0
Amounts reclassified from AOCI	0.0	0.0	0.0	0.0
Other comprehensive loss	25.1	168.5	(16.1)	350.0
Ending balance	42.9	(36.9)	42.9	(36.9)
Net Funded Status of Benefit Plans
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	(8.8)	(10.2)	(8.8)	(10.2)
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 loss	0.0	0.0	0.0	0.0
Ending balance	$ (8.8)	$ (10.2)	$ (8.8)	$ (10.2)

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