Marketable Securities (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Number	Dec. 31, 2016 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Available-For-Sale Securities Number of Securities Owned | Number	19
Available-For-Sale Securities Number Of Securities Owned Unrealized Losses | Number	18
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | $	$ 17	$ 10
Available-for-sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax | $	$ 100