CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Convertible Preferred Stock [Member]	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Beginning Balance at Dec. 31, 2017	$ 21,544	$ 20,966	$ 3	$ 633,436	$ (7)	$ (632,854)
Beginning Balance, shares at Dec. 31, 2017		18,000	34,291,000
Adjustment to issuance cost related to common stock, preferred stock, and warrants issued in prior year	126	$ 32		94
Issuance of common stock through controlled equity offering facilities, net of issuance costs	5,991		$ 1	5,990
Issuance of common stock through controlled equity offering facilities, net of issuance costs, shares			3,009,000
Issuance of common stock from vesting of restricted stock awards	83			83
Issuance of common stock from vesting of restricted stock awards, shares			21,000
Issuance of common stock under employee stock purchase plans	$ 139			139
Issuance of common stock under employee stock purchase plans, shares	104,099		104,000
Issuance of common stock pursuant to stock option exercises	$ 164			164
Issuance of common stock pursuant to stock option exercises, shares			49,000
Stock-based compensation expenses	2,554			2,554
Net loss	(26,615)					(26,615)
Unrealized gain on available-for-sale securities	7				7
Ending Balance at Dec. 31, 2018	3,993	$ 20,998	$ 4	642,460		(659,469)
Ending Balance, shares at Dec. 31, 2018		18,000	37,474,000
Issuance of common stock, preferred stock, net of issuance cost	44,604	$ 12,533	$ 6	32,065
Issuance of common stock, preferred stock, net of issuance cost, shares		25,000	61,333,000
Issuance of common stock upon conversion of preferred stock		$ (21,762)	$ 1	21,761
Issuance of common stock upon conversion of preferred stock, shares		(23,000)	11,950,165
Issuance of common stock through controlled equity offering facilities, net of issuance costs	464			464
Issuance of common stock through controlled equity offering facilities, net of issuance costs, shares			398,000
Issuance of common stock from vesting of restricted stock awards	54			54
Issuance of common stock from vesting of restricted stock awards, shares			104,000
Issuance of common stock under employee stock purchase plans	$ 63			63
Issuance of common stock under employee stock purchase plans, shares	133,322		134,000
Stock-based compensation expenses	$ 1,695			1,695
Net loss	(23,330)
Net loss | ASU 2018-07 [Member]	(23,331)					(23,331)
Unrealized gain on available-for-sale securities	1				1
Ending Balance at Dec. 31, 2019	$ 27,543	$ 11,769	$ 11	$ 698,562	$ 1	$ (682,800)
Ending Balance, shares at Dec. 31, 2019		20,000	111,393,000