Condensed Consolidated Statements of Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (6,482)	$ (5,877)	$ (11,524)	$ (10,015)
Other comprehensive income:
Unrealized gain on available-for-sale securities	8		4	1
Total comprehensive loss	$ (6,474)	$ (5,877)	$ (11,520)	$ (10,014)

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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