Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (12,960)	$ (8,973)	$ (9,238)
Unrealized gain (loss) on available-for-sale and long-term marketable securities:
Unrealized gain (loss) arising during holding period, net of tax benefit of $56, $0 and $32 for years ended December 31, 2017, 2016 and 2015, respectively	97	(15)	52
Other comprehensive gain (loss)	97	(15)	52
Total comprehensive loss	$ (12,863)	$ (8,988)	$ (9,186)