Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Shares [Member]	Additional Paid-In Capital [Member]	Accumulated Deficit [Member]
Beginning Balance at Dec. 31, 2018	$ 79,682	$ 3	$ 613,721	$ (534,042)
Beginning Balance, shares at Dec. 31, 2018		2,684
Issuance of common stock under stock plans, shares		3
Stock-based compensation expense	2,723		2,723
Net loss	(19,488)			(19,488)
Ending Balance at Mar. 31, 2019	62,917	$ 3	616,444	(553,530)
Ending Balance, shares at Mar. 31, 2019		2,687
Beginning Balance at Dec. 31, 2019	23,167	$ 3	627,291	(604,127)
Beginning Balance, shares at Dec. 31, 2019		3,466
Issuance of common stock under stock plans	1		1
Issuance of common stock under stock plans, shares		23
Issuance of common stock and prefunded warrants under a concurrent private placement and registered direct offering, less issuance costs	15,931	$ 4	15,927
Issuance of common stock and prefunded warrants under a concurrent private placement and registered direct offering, less issuance costs, shares		3,650
Issuance of common stock from warrant exercise, shares		120
Stock-based compensation expense	480		480
Net loss	(12,124)			(12,124)
Ending Balance at Mar. 31, 2020	$ 27,455	$ 7	$ 643,699	$ (616,251)
Ending Balance, shares at Mar. 31, 2020		7,259