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