Cash and Cash Equivalents and Investments in Marketable Securities (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Investment Income, Interest	$ 2,590	$ 819
Maximum term to maturity from date of purchase (years)	1 year
Available-for-sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment Income, Interest	$ 0	$ 0