Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Total
Stockholders' Equity Attributable to Parent, Beginning Balance at Dec. 31, 2017	$ 1	$ 194,016	$ (183,907)	$ 10,110
Beginning Balance, (in shares) at Dec. 31, 2017	1,370,992
Share-based compensation expense		456		456
Fair value of derivative warrant liability reclassified to additional paid-in capital		1		1
Issuance of common stock upon exercise of warrants		1,900		1,900
Issuance of common stock upon exercise of warrants (in shares)	1,050,918
Issuance of common stock in public offerings		3,338		3,338
Issuance of common stock in public offerings (in shares)	1,652,396
Issuance of common stock under ESPP		3		3
Issuance of common stock under ESPP (in shares)	188
Fractional shares issued due to reverse stock split	2,733
Issuance of common stock to 401(k) plan		6		6
Issuance of common stock to 401(k) plan (in shares)	440
Net loss			(17,723)	(17,723)
Stockholders' Equity Attributable to Parent, Ending Balance at Jun. 30, 2018	$ 1	199,720	(201,630)	(1,909)
Ending Balance, (in shares) at Jun. 30, 2018	4,077,667
Stockholders' Equity Attributable to Parent, Beginning Balance at Dec. 31, 2017	$ 1	194,016	(183,907)	10,110
Beginning Balance, (in shares) at Dec. 31, 2017	1,370,992
Stockholders' Equity Attributable to Parent, Ending Balance at Dec. 31, 2018	$ 1	223,440	(207,330)	$ 16,111
Ending Balance, (in shares) at Dec. 31, 2018	9,309,255			9,309,255
Stockholders' Equity Attributable to Parent, Beginning Balance at Mar. 31, 2018	$ 1	197,013	(188,691)	$ 8,323
Beginning Balance, (in shares) at Mar. 31, 2018	1,562,284
Share-based compensation expense		150		150
Fair value of derivative warrant liability reclassified to additional paid-in capital		1		1
Issuance of common stock upon exercise of warrants		1,900		1,900
Issuance of common stock upon exercise of warrants (in shares)	1,050,918
Issuance of common stock in public offerings		656		656
Issuance of common stock in public offerings (in shares)	1,461,732
Issuance of common stock under ESPP (in shares)	2,733
Net loss			(12,939)	(12,939)
Stockholders' Equity Attributable to Parent, Ending Balance at Jun. 30, 2018	$ 1	199,720	(201,630)	(1,909)
Ending Balance, (in shares) at Jun. 30, 2018	4,077,667
Stockholders' Equity Attributable to Parent, Beginning Balance at Dec. 31, 2018	$ 1	223,440	(207,330)	$ 16,111
Beginning Balance, (in shares) at Dec. 31, 2018	9,309,255			9,309,255
Share-based compensation expense		135		$ 135
Issuance of common stock upon vesting of restricted stock units (in shares)	1,500
Issuance of common stock under ESPP		1		1
Issuance of common stock under ESPP (in shares)	1,065
Net loss			(5,779)	(5,779)
Stockholders' Equity Attributable to Parent, Ending Balance at Jun. 30, 2019	$ 1	223,576	(213,109)	$ 10,468
Ending Balance, (in shares) at Jun. 30, 2019	9,311,820			9,311,820
Stockholders' Equity Attributable to Parent, Beginning Balance at Mar. 31, 2019	$ 1	223,508	(209,965)	$ 13,544
Beginning Balance, (in shares) at Mar. 31, 2019	9,311,070
Share-based compensation expense		68		68
Issuance of common stock upon vesting of restricted stock units (in shares)	750
Net loss			(3,144)	(3,144)
Stockholders' Equity Attributable to Parent, Ending Balance at Jun. 30, 2019	$ 1	$ 223,576	$ (213,109)	$ 10,468
Ending Balance, (in shares) at Jun. 30, 2019	9,311,820			9,311,820