Fair Value Measurements (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Increase in fair value of marketable securities	$ 1.0	$ 0.9	$ 2.6	$ 1.4
Non-financial assets and liabilities measured at fair value on a nonrecurring basis	$ 0.0		$ 0.0		$ 0.0