CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Series D Preferred Stock	Series E Preferred Stock	At the market offering	Private placement	Preferred shares	Preferred shares Series D Preferred Stock	Preferred shares Series E Preferred Stock	Common shares	Common shares Series D Preferred Stock	Common shares Series E Preferred Stock	Common shares At the market offering	Common shares Private placement	ATM proceeds receivable	ATM proceeds receivable At the market offering	Additional paid-in capital	Additional paid-in capital Series D Preferred Stock	Additional paid-in capital Series E Preferred Stock	Additional paid-in capital At the market offering	Additional paid-in capital Private placement	Accumulated other comprehensive income (loss)	Accumulated deficit
Balance at beginning of period (in shares) at Dec. 31, 2019						11			55,212
Balance at beginning of period at Dec. 31, 2019	$ 51,352					$ 0			$ 55					$ 0		$ 227,468					$ (1,995)	$ (174,176)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(22,830)																					(22,830)
Foreign currency translation adjustment	1,462																				1,462	0
Unrealized gain (loss) on investments	(249)																				(249)
Stock based compensation expense	3,916															3,916
Issuance of common stock resulting from exercise of stock options (in shares)									139
Issuance of common stock resulting from exercise of stock options	271															271
Issuance of common stock resulting from purchase of employee stock purchase plan shares (in shares)									47
Issuance of common stock resulting from purchase of employee stock purchase plan shares	145															145
Preferred stock conversion (in shares)							3			3,000
Preferred stock conversion value		$ 0								$ (3)							$ 3
Issuance of common stock pursuant to offering (in shares)												3,571
Issuance of common stock pursuant to offering value				$ 9,624								$ 4							$ 9,620
Balance at end of period (in shares) at Sep. 30, 2020						14			55,971
Balance at end of period at Sep. 30, 2020	43,691					$ 0			$ 56					0		241,423					(782)	(197,006)
Balance at beginning of period (in shares) at Jun. 30, 2020						14			52,399
Balance at beginning of period at Jun. 30, 2020	46,672					$ 0			$ 52					0		230,407					(1,064)	(182,723)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(14,283)																					(14,283)
Foreign currency translation adjustment	632																				632
Unrealized gain (loss) on investments	(350)																				(350)
Stock based compensation expense	1,396															1,396
Issuance of common stock pursuant to offering (in shares)												3,571
Issuance of common stock pursuant to offering value				9,624								$ 4							9,620
Balance at end of period (in shares) at Sep. 30, 2020						14			55,971
Balance at end of period at Sep. 30, 2020	43,691					$ 0			$ 56					0		241,423					(782)	(197,006)
Balance at beginning of period (in shares) at Dec. 31, 2020						14			56,003
Balance at beginning of period at Dec. 31, 2020	31,027					$ 0			$ 56					0		242,672					(295)	(211,406)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(36,206)																					(36,206)
Foreign currency translation adjustment	911																				911
Stock based compensation expense	3,895															3,895
Issuance of common stock resulting from exercise of stock options (in shares)									412
Issuance of common stock resulting from exercise of stock options	858															858
Issuance of common stock resulting from purchase of employee stock purchase plan shares (in shares)									40
Issuance of common stock resulting from purchase of employee stock purchase plan shares	96															96
Issuance of common stock resulting from exercise of warrants (in shares)									1,391
Issuance of common stock resulting from exercise of warrants	837								$ 1							836
Preferred stock conversion (in shares)						4		5	3,812		5,000
Preferred stock conversion value	0		$ 0						$ 4		$ (5)					(4)		$ 5
Issuance of common stock pursuant to offering (in shares)												7,558	7,290
Issuance of common stock pursuant to offering value				33,522	$ 18,913							$ 8	$ 8		$ (1,814)				35,328	$ 18,905
Balance at end of period (in shares) at Sep. 30, 2021						16			71,505
Balance at end of period at Sep. 30, 2021	53,853					$ 0			$ 72					(1,814)		302,591					616	(247,612)
Balance at beginning of period (in shares) at Jun. 30, 2021						16			66,679
Balance at beginning of period at Jun. 30, 2021	46,721					$ 0			$ 67					0		277,496					234	(231,076)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(16,536)																					(16,536)
Foreign currency translation adjustment	382																				382
Stock based compensation expense	1,370															1,370
Issuance of common stock resulting from exercise of stock options (in shares)									272
Issuance of common stock resulting from exercise of stock options	566															566
Issuance of common stock pursuant to offering (in shares)												4,554
Issuance of common stock pursuant to offering value				$ 21,350								$ 5			$ (1,814)				$ 23,159
Balance at end of period (in shares) at Sep. 30, 2021						16			71,505
Balance at end of period at Sep. 30, 2021	$ 53,853					$ 0			$ 72					$ (1,814)		$ 302,591					$ 616	$ (247,612)