Income Taxes (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount	$ 99.8
Deferred Tax Assets, Valuation Allowance	5.5	$ 0.0
Unrecognized tax benefit that, if recognized, would favorably affect our effective income tax rate	31.9
Accrued interest related to uncertain tax positions	6.1	4.9
Tax benefits from stock-based compensation plans recognized in net income	2.5
Tax benefits from stock-based compensation plans	$ 0.0	$ 27.2	$ 0.3