Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Expected income tax expense	$ 93.4	$ 109.6	$ 59.3
Tax difference related to investment disposals and maturities	(9.2)	(14.8)	(9.2)
Stock-based compensation windfall benefit	(2.1)	(3.0)	(2.3)
Dividend received deduction	(1.1)	(1.3)	(1.2)
Tax-exempt interest	(0.2)	(0.3)	(0.4)
Nondeductible expenses	1.8	1.7	1.6
Effect of changes in the tax law and rates		1.2	(4.3)
Other, net	0.2
Total income tax expense	$ 82.8	$ 93.1	$ 43.5
Effective tax rate	18.60%	17.80%	15.40%