Income Taxes (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Deferred Tax Assets, Operating Loss Carryforwards	$ 3.4
U.S. federal statutory rate	21.00%	35.00%	35.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount	$ 99.8
Deferred Tax Assets, Valuation Allowance	0.0	$ 5.5
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount	5.5	(5.5)
Unrecognized tax benefit that, if recognized, would favorably affect our effective income tax rate	38.7
Accrued interest related to uncertain tax positions	7.5	6.1
Tax benefits from stock-based compensation plans	$ 0.0	$ 0.0	$ 27.2