Statements of Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (4,578)	$ (6,270)
Unrealized gain (loss) on available-for-sale and long-term marketable securities:
Unrealized gain (loss) arising during holding period for three months ended March 31, 2019 and 2018, respectively	118	(65)
Less: Reclassification adjustment for gains included in net loss	(373)	0
Other comprehensive loss	(255)	(65)
Total comprehensive loss	$ (4,833)	$ (6,335)

Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners.

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Amount after tax, before reclassification adjustments, of unrealized holding gain (loss) on available-for-sale securities.

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