Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Tax Disclosure [Abstract]
Expected income tax expense	$ 8.7	$ 30.3	$ 109.5
Nondeductible expenses	3.8	3.1	2.3
Tax difference related to investment disposals and maturities	(1.3)	(1.4)	(4.6)
Stock-based compensation windfall benefit	(1.1)	(3.2)	(2.6)
Change in uncertain tax positions	(0.9)
Current year federal tax credits	(0.7)	(0.4)	(0.5)
Dividend received deduction	(0.5)	(1.0)	(1.2)
Prior years' federal research tax credits			(1.7)
Other, net	(0.4)	(0.2)	0.1
Total income tax expense	$ 7.6	$ 27.2	$ 101.3
Effective tax rate	18.50%	18.90%	19.40%