CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Preferred Stock Series B convertible preferred stock	Preferred Stock Series C convertible preferred stock	Preferred Stock Series C-1 convertible preferred stock	Preferred Stock Series D convertible preferred stock	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive (Loss) Income	Accumulated Deficit
Stockholders' Equity, Beginning Balance at Dec. 31, 2015	$ (35,179)	$ 28,220	$ 6,452	$ 2,156	$ 40,688	$ 3	$ 15,482	$ 0	$ (50,664)
Shares, Outstanding, Beginning Balance at Dec. 31, 2015		3,963,000	1,593,000	474,000	5,491,000	2,630,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued under employee stock plans	31						31
Shares issued under employee stock plans (in shares)						23,000
Repurchase of shares	$ (1)						(1)
Repurchase of shares (in shares)	(1,457)					(2,000)
Stock-based compensation	$ 1,160						1,160
Net loss	(4,259)								(4,259)
Stockholders' Equity, Ending Balance at Dec. 31, 2016	(38,248)	$ 28,220	$ 6,452	$ 2,156	$ 40,688	$ 3	16,672	0	(54,923)
Shares, Outstanding, Ending Balance at Dec. 31, 2016		3,963,000	1,593,000	474,000	5,491,000	2,651,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued for public offerings, net of underwriters' fees	292,537					$ 9	292,528
Stock issued during period (in shares)						9,021,000
Shares issued under employee stock plans	$ 979						979
Shares issued under employee stock plans (in shares)						199,000
Repurchase of shares (in shares)	0
Total offering costs	$ (4,228)						(4,228)
Conversion of preferred stock	77,516	$ (28,220)	$ (6,452)	$ (2,156)	$ (40,688)	$ 12	77,504
Conversion of preferred stock (in shares)		(3,963,000)	(1,593,000)	(474,000)	(5,491,000)	11,521,000
Warrants exercised	536						536
Warrants exercised (in shares)						399,000
Reclassification of warrants	4,607						4,607
Forfeiture rate adjustment	0						41		(41)
Stock-based compensation	4,378						4,378
Comprehensive income (loss)	(426)							(426)
Net loss	(30,070)								(30,070)
Stockholders' Equity, Ending Balance at Dec. 31, 2017	307,581	$ 0	$ 0	$ 0	$ 0	$ 24	393,017	(426)	(85,034)
Shares, Outstanding, Ending Balance at Dec. 31, 2017		0	0	0	0	23,791,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares issued for public offerings, net of underwriters' fees	227,476					$ 3	227,473
Stock issued during period (in shares)						2,530,000
Shares issued under employee stock plans	$ 2,869						2,869
Shares issued under employee stock plans (in shares)	583,660					584,000
Repurchase of shares (in shares)	0
Total offering costs	$ (145)						(145)
Warrants exercised	76						76
Warrants exercised (in shares)						17,000
Stock-based compensation	9,961						9,961
Comprehensive income (loss)	203							203
Net loss	(61,656)								(61,656)
Stockholders' Equity, Ending Balance at Dec. 31, 2018	$ 486,365	$ 0	$ 0	$ 0	$ 0	$ 27	$ 633,251	$ (223)	$ (146,690)
Shares, Outstanding, Ending Balance at Dec. 31, 2018		0	0	0	0	26,922,000