Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income [Member]	Accumulated Deficit [Member]	Total
Balance at Dec. 31, 2017	$ 109,104	$ 7	$ 157,395		$ (269,920)	$ (3,414)
Balance, shares at Dec. 31, 2017	1,656,838	7,082,836
Conversion of Series A preferred stock to common stock	$ (391)		391
Conversion of Series A preferred stock to common stock, shares	(1,602,099)	363,036
Conversion of Series B preferred stock to common stock	$ (4,020)	$ 1	4,019
Conversion of Series B preferred stock to common stock, shares	(47,689)	794,820
Conversion of Series E preferred stock to common stock	$ (104,693)	$ 7	104,686
Conversion of Series E preferred stock to common stock, shares	(7,050)	7,050,000
Exchange of Series F preferred stock and dividends to common stock		$ 1	13,060			13,061
Exchange of Series F preferred stock and dividends to common stock, shares		1,003,393
Extinguishment of warrant liability			3,045			3,045
Extinguishment of warrant liability, shares		151,871
Stock-based compensation expense			7,445			7,445
Stock-based compensation expense, shares
Deemed dividend - accretion of discount on Series F preferred stock			(698)			(698)
Cumulative dividends on Series F preferred stock			(191)			(191)
Series F preferred stock dividends paid in common stock			306			306
Series F preferred stock dividends paid in common stock, shares		11,708
Net loss					(11,777)	(11,777)
Balance at Mar. 31, 2018		$ 16	289,458		(281,697)	7,777
Balance, shares at Mar. 31, 2018		16,457,664
Balance at Dec. 31, 2017	$ 109,104	$ 7	157,395		(269,920)	(3,414)
Balance, shares at Dec. 31, 2017	1,656,838	7,082,836
Deemed dividend - accretion of discount on Series F preferred stock						(698)
Cumulative dividends on Series F preferred stock						(191)
Net loss						(25,777)
Balance at Jun. 30, 2018		$ 20	390,538		(295,697)	94,861
Balance, shares at Jun. 30, 2018		21,445,370
Balance at Mar. 31, 2018		$ 16	289,458		(281,697)	7,777
Balance, shares at Mar. 31, 2018		16,457,664
Stock-based compensation expense			8,344			8,344
Stock-based compensation expense, shares		175,887
Deemed dividend - accretion of discount on Series F preferred stock
Cumulative dividends on Series F preferred stock
Proceeds received from issuance of common stock, net of issuance costs of $2,782		$ 4	92,672			92,676
Proceeds received from issuance of common stock, net of issuance costs of $2,782, shares		4,791,819
Stock option exercises			64			64
Stock option exercises shares		20,000
Net loss					(14,000)	(14,000)
Balance at Jun. 30, 2018		$ 20	390,538		(295,697)	94,861
Balance, shares at Jun. 30, 2018		21,445,370
Balance at Dec. 31, 2018		$ 21	414,840	36	(342,864)	72,033
Balance, shares at Dec. 31, 2018		21,447,088
Stock-based compensation expense			10,327			10,327
Stock-based compensation expense, shares
Deemed dividend - accretion of discount on Series F preferred stock
Cumulative dividends on Series F preferred stock
Series F preferred stock dividends paid in common stock
Stock option exercises		$ 1	528			529
Stock option exercises shares		283,250
Vesting of restricted stock units
Vesting of restricted stock units, shares		100,912
Shares withheld for tax withholding			(740)			(740)
Shares withheld for tax withholding, shares		(82,011)
Other comprehensive income				17		17
Net loss					(25,573)	(25,573)
Balance at Mar. 31, 2019		$ 22	424,955	53	(368,437)	56,593
Balance, shares at Mar. 31, 2019		21,749,239
Balance at Dec. 31, 2018		$ 21	414,840	36	(342,864)	72,033
Balance, shares at Dec. 31, 2018		21,447,088
Deemed dividend - accretion of discount on Series F preferred stock
Cumulative dividends on Series F preferred stock
Shares withheld for tax withholding						598
Net loss						(48,365)
Balance at Jun. 30, 2019		$ 25	461,491	79	(391,229)	70,366
Balance, shares at Jun. 30, 2019		25,218,606
Balance at Mar. 31, 2019		$ 22	424,955	53	(368,437)	56,593
Balance, shares at Mar. 31, 2019		21,749,239
Stock-based compensation expense			8,618			8,618
Stock-based compensation expense, shares
Deemed dividend - accretion of discount on Series F preferred stock
Cumulative dividends on Series F preferred stock
Proceeds received from issuance of common stock, net of issuance costs of $2,782		$ 3	27,945			27,948
Proceeds received from issuance of common stock, net of issuance costs of $2,782, shares		3,418,918
Stock option exercises
Stock option exercises shares		9,167
Vesting of restricted stock units
Vesting of restricted stock units, shares		51,440
Shares withheld for tax withholding			(62)			(62)
Shares withheld for tax withholding, shares		(17,418)
Shares issued under the ESPP			35			35
Shares issued under the ESPP, shares		7,260
Other comprehensive income				26		26
Net loss					(22,792)	(22,792)
Balance at Jun. 30, 2019		$ 25	$ 461,491	$ 79	$ (391,229)	$ 70,366
Balance, shares at Jun. 30, 2019		25,218,606