Fair Value Measurements (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
(Decrease) increase in fair value of marketable securities	$ (0.1)	$ 0.9
Non-financial assets and liabilities measured at fair value on a nonrecurring basis	$ 0.0		$ 0.0