Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance			$ 1,790.1
Other comprehensive income (loss) before reclassifications	$ (29.3)	$ 54.3	(65.0)	$ 97.3
Amounts reclassified from AOCI	4.0	(5.2)	5.6	0.5
Other comprehensive (loss) income	(25.3)	49.1	(59.4)	97.8
Ending balance	1,796.4	1,715.8	1,796.4	1,715.8
Pretax reclassification amounts from AOCI, tax	(1.0)	1.4	(1.5)	(0.1)
Total
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	321.0	268.4	355.1	219.7
Ending balance	295.7	317.5	295.7	317.5
Net Unrealized Investment Gains (Losses) on Securities
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	332.2	279.1	366.3	230.4
Other comprehensive income (loss) before reclassifications	(29.3)	54.3	(65.0)	97.3
Amounts reclassified from AOCI	4.0	(5.2)	5.6	0.5
Other comprehensive (loss) income	(25.3)	49.1	(59.4)	97.8
Ending balance	306.9	328.2	306.9	328.2
Pretax reclassification amounts from accumulated other comprehensive income (loss)	(5.0)	6.6	(7.1)	(0.6)
Net Funded Status of Benefit Plans
Net unrealized investment gains (losses) on fixed maturity securities, net of tax
Beginning balance	(11.2)	(10.7)	(11.2)	(10.7)
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) income	0.0	0.0	0.0	0.0
Ending balance	$ (11.2)	$ (10.7)	$ (11.2)	$ (10.7)