Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Expected income tax expense	$ 30.3	$ 109.5	$ 93.4
Nondeductible expenses	3.1	2.3	1.8
Stock-based compensation windfall benefit	(3.2)	(2.6)	(2.1)
Tax difference related to investment disposals and maturities	(1.4)	(4.6)	(9.2)
Dividend received deduction	(1.0)	(1.2)	(1.1)
Current year federal tax credits	(0.4)	(0.5)
Prior years' federal research tax credits		(1.7)
Other, net	(0.2)	0.1	0.0
Total income tax expense	$ 27.2	$ 101.3	$ 82.8
Effective tax rate	18.90%	19.40%	18.60%