Consolidated Statements of Stockholders' Equity - Parenthetical - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Stockholders' Equity [Abstract]
Unrealized gain (loss) on marketable securities tax expense (benefit)	$ (27)	$ 5	$ 1
Loss on derivative securities tax benefit	(171)	14	0
Foreign currency translation tax expense (benefit)	$ 1	$ (5)	$ 17