Condensed Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Common Stock	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]	Total
Balance at Dec. 31, 2017	$ 17	$ 84,202	$ (51)	$ (38,173)	$ 45,995
Balance, shares at Dec. 31, 2017	16,819
Issuance of shares upon exercise of stock options		112			112
Issuance of shares upon exercise of stock options, shares	38
Issuance of shares under employee stock purchase plan		188			188
Issuance of shares under employee stock purchase plan, shares	12
Stock-based compensation expense		3,420			3,420
Unrealized gain on available-for-sale securities			48		48
Net loss				(8,668)	(8,668)
Balance at Mar. 31, 2018	$ 17	87,922	(3)	(46,841)	41,095
Balance, shares at Mar. 31, 2018	16,869
Balance at Dec. 31, 2018	$ 21	142,032	(1)	(75,718)	$ 66,334
Balance, shares at Dec. 31, 2018	20,593				20,593
Issuance of shares upon exercise of stock options		272			$ 272
Issuance of shares upon exercise of stock options, shares	99
Issuance of shares under employee stock purchase plan		222			222
Issuance of shares under employee stock purchase plan, shares	18
Stock-based compensation expense		2,361			2,361
Unrealized gain on available-for-sale securities			3		3
Net loss				(10,078)	(10,078)
Balance at Mar. 31, 2019	$ 21	$ 144,887	$ 2	$ (85,796)	$ 59,114
Balance, shares at Mar. 31, 2019	20,710				20,710