Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance			$ 1,807.4
Other comprehensive loss before reclassifications	$ (188.2)	$ (29.3)	(725.7)	$ (65.0)
Amounts reclassified from AOCI	11.9	4.0	38.3	5.6
Other comprehensive loss	(176.3)	(25.3)	(687.4)	(59.4)
Ending balance	1,076.6	1,796.4	1,076.6	1,796.4
Pretax reclassification amounts from AOCI, tax	(3.2)	(1.0)	(10.2)	(1.5)
Total
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	(230.6)	321.0	280.5	355.1
Ending balance	(406.9)	295.7	(406.9)	295.7
Net Unrealized Investment Gains (Losses) on Securities
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	(220.4)	332.2	290.7	366.3
Other comprehensive loss before reclassifications	(188.2)	(29.3)	(725.7)	(65.0)
Amounts reclassified from AOCI	11.9	4.0	38.3	5.6
Other comprehensive loss	(176.3)	(25.3)	(687.4)	(59.4)
Ending balance	(396.7)	306.9	(396.7)	306.9
Pretax reclassification amounts from accumulated other comprehensive income (loss)	(15.0)	(5.0)	(48.4)	(7.1)
Net Funded Status of Benefit Plans
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	(10.2)	(11.2)	(10.2)	(11.2)
Other comprehensive 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	$ (10.2)	$ (11.2)	$ (10.2)	$ (11.2)