Cash and Cash Equivalents and Investments in Marketable Securities (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Interest income	$ 3,736	$ 2,491	$ 3,391
Available-for-sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Interest income	$ 300	$ 800	$ 1,900