Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Common Stock Private Placement [Member]	Common Stock Warrants to purchase common stock [Member]	Common Stock Rights Offering [Member]	Common Stock	Additional Paid-in Capital [Member] Private Placement [Member]	Additional Paid-in Capital [Member] Warrants to purchase common stock [Member]	Additional Paid-in Capital [Member] Rights Offering [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Loss [Member] Private Placement [Member]	Accumulated Other Comprehensive Loss [Member] Warrants to purchase common stock [Member]	Accumulated Other Comprehensive Loss [Member] Rights Offering [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member] Private Placement [Member]	Accumulated Deficit [Member] Warrants to purchase common stock [Member]	Accumulated Deficit [Member] Rights Offering [Member]	Accumulated Deficit [Member]	Private Placement [Member]	Warrants to purchase common stock [Member]	Rights Offering [Member]	Total
Balance at Dec. 31, 2016				$ 13				$ 37,898				$ (7)				$ (12,606)				$ 25,298
Balance, shares at Dec. 31, 2016				13,315,000
Shares issued	$ 3				$ 34,840												$ 34,843
Shares issued, shares	2,820,000
Shares issued upon exercise of stock options				$ 1		$ 50		488										$ 50		489
Shares issued upon exercise of stock options, shares		522,000		162,000
Stock-based compensation expense								10,926												10,926
Unrealized loss on available-for-sale securities, net of tax												(44)								(44)
Net loss																(25,567)				(25,567)
Balance at Dec. 31, 2017				$ 17				84,202				(51)				(38,173)				$ 45,995
Balance, shares at Dec. 31, 2017				16,819,000																16,819,000
Shares issued			$ 4				$ 44,782												$ 44,786
Shares issued, shares			3,581,000
Shares issued upon exercise of stock options								498												$ 498
Shares issued upon exercise of stock options, shares		24,000		145,000														36,224		144,508
Issuance of shares under employee stock plans								327												$ 327
Issuance of shares under employee stock plans, shares				24,000
Stock-based compensation expense								12,338												12,338
Tax payments related to shares withheld for vested restricted stock units								(115)												(115)
Unrealized loss on available-for-sale securities, net of tax												50								50
Net loss																(37,545)				(37,545)
Balance at Dec. 31, 2018				$ 21				$ 142,032				$ (1)				$ (75,718)				$ 66,334
Balance, shares at Dec. 31, 2018				20,593,000																20,593,000