Condensed Consolidated Statements Stockholders' (Deficit) Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]
Balance at Dec. 31, 2017	$ 1,340	$ 36	$ 267,421	$ (266,117)
Balance (in shares) at Dec. 31, 2017		35,948,900
Stock-based compensation expense	1,381		1,381
Issuance of common stock from vesting of restricted stock, exercise of stock options and employee stock purchase plan	5		5
Issuance of common stock from vesting of restricted stock, exercise of stock options and employee stock purchase plan (in shares)		70,983
Prior year accumulated deficit adjustment from ASC 606 implementation	99			99
Net loss	(12,913)			(12,913)
Balance at Mar. 31, 2018	(10,088)	$ 36	268,807	(278,931)
Balance (in shares) at Mar. 31, 2018		36,019,883
Balance at Dec. 31, 2017	1,340	$ 36	267,421	(266,117)
Balance (in shares) at Dec. 31, 2017		35,948,900
Net loss	(25,237)
Balance at Jun. 30, 2018	31,983	$ 43	323,195	(291,255)
Balance (in shares) at Jun. 30, 2018		43,472,411
Balance at Mar. 31, 2018	(10,088)	$ 36	268,807	(278,931)
Balance (in shares) at Mar. 31, 2018		36,019,883
Stock-based compensation expense	3,898		3,898
Issuance of common stock from vesting of restricted stock, exercise of stock options and employee stock purchase plan	1,119		1,119
Issuance of common stock from vesting of restricted stock, exercise of stock options and employee stock purchase plan (in shares)		382,114
Share issuance (in shares)		55,414
Issuance of common stock from secondary offering, net	49,378	$ 7	49,371
Issuance of common stock from secondary offering, net (in shares)		7,015,000
Net loss	(12,324)			(12,324)
Balance at Jun. 30, 2018	31,983	$ 43	323,195	(291,255)
Balance (in shares) at Jun. 30, 2018		43,472,411
Balance at Dec. 31, 2018	$ 11,387	$ 44	328,514	(317,171)
Balance (in shares) at Dec. 31, 2018	44,175,441	44,175,441
Stock-based compensation expense	$ 2,033		2,033
Issuance of common stock from vesting of restricted stock, exercise of stock options and employee stock purchase plan (in shares)		163,802
Change in fair value of warrants upon modification	147		147
Net loss	(15,147)			(15,147)
Balance at Mar. 31, 2019	(1,580)	$ 44	330,694	(332,318)
Balance (in shares) at Mar. 31, 2019		44,339,243
Balance at Dec. 31, 2018	$ 11,387	$ 44	328,514	(317,171)
Balance (in shares) at Dec. 31, 2018	44,175,441	44,175,441
Net loss	$ (30,794)
Balance at Jun. 30, 2019	$ (15,620)	$ 44	332,301	(347,965)
Balance (in shares) at Jun. 30, 2019	44,535,572	44,535,572
Balance at Mar. 31, 2019	$ (1,580)	$ 44	330,694	(332,318)
Balance (in shares) at Mar. 31, 2019		44,339,243
Stock-based compensation expense	1,277		1,277
Issuance of common stock from vesting of restricted stock, exercise of stock options and employee stock purchase plan	330		330
Issuance of common stock from vesting of restricted stock, exercise of stock options and employee stock purchase plan (in shares)		196,329
Net loss	(15,647)			(15,647)
Balance at Jun. 30, 2019	$ (15,620)	$ 44	$ 332,301	$ (347,965)
Balance (in shares) at Jun. 30, 2019	44,535,572	44,535,572