Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Expected income tax expense	$ 59.3	$ 102.5	$ 23.1
Effect of the enactment of the Tax Cuts and Jobs Act	(4.3)	(3.9)
Tax difference related to investment disposals and maturities	(9.2)	(12.7)	(20.7)
Stock-based compensation windfall benefit	(2.3)	(5.3)
Nondeductible expenses	1.6	1.0	1.0
Dividend received deduction	(1.2)	(3.2)	(3.3)
Tax-exempt interest	(0.4)	(0.9)	(1.1)
Change in liability for uncertain tax positions		(0.5)
Other, net		(0.2)
Total income tax expense (benefit)	$ 43.5	$ 76.8	$ (1.0)
Effective tax rate	15.40%	26.20%	(1.50%)