Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Balance at Dec. 31, 2017	$ 30,774		$ 6	$ 204,262		$ (173,494)
Balance (in shares) at Dec. 31, 2017		3,680	6,081,230
Opening balance adjustment - adoption of ASC 606 at Dec. 31, 2017	207					207
Balance at Dec. 31, 2017	30,981		$ 6	204,262		(173,287)
Stock-based compensation expense	606			606
Issuance of common stock for secondary public offering, net of issuance costs	106,762		$ 4	106,758
Issuance of common stock for secondary public offering, net of issuance costs, shares			3,382,352
Issuance of common stock upon exercise of warrants	9,545		$ 2	9,543
Issuance of common stock upon exercise of warrants, shares			1,735,419
Conversion of preferred stock to common stock, shares		(3,680)	736,000
Issuance of common stock from option exercises, shares			59
Conversion of redeemable convertible notes to common stock, value	3			3
Conversion of redeemable convertible notes to common stock, shares			21
Unrealized (loss) gain on available-for-sale securities	(4)				$ (4)
Net loss	(5,042)					(5,042)
Balance at Mar. 31, 2018	142,851		$ 12	321,172	(4)	(178,329)
Balance (in shares) at Mar. 31, 2018			11,935,081
Balance at Dec. 31, 2017	30,774		$ 6	204,262		(173,494)
Balance (in shares) at Dec. 31, 2017		3,680	6,081,230
Opening balance adjustment - adoption of ASC 606 at Dec. 31, 2017	207					207
Balance at Dec. 31, 2017	30,981		$ 6	204,262		(173,287)
Unrealized (loss) gain on available-for-sale securities	(24)
Net loss	(19,218)
Balance at Sep. 30, 2018	129,951		$ 12	322,468	(24)	(192,505)
Balance (in shares) at Sep. 30, 2018			11,942,729
Balance at Mar. 31, 2018	142,851		$ 12	321,172	(4)	(178,329)
Balance (in shares) at Mar. 31, 2018			11,935,081
Stock-based compensation expense	550			550
Issuance of common stock from option exercises	36			36
Issuance of common stock from option exercises, shares			7,648
Unrealized (loss) gain on available-for-sale securities	8				8
Net loss	(6,482)					(6,482)
Balance at Jun. 30, 2018	136,963		$ 12	321,758	4	(184,811)
Balance (in shares) at Jun. 30, 2018			11,942,729
Issuance of common stock from option exercises	710			710
Unrealized (loss) gain on available-for-sale securities	(28)				(28)
Net loss	(7,694)					(7,694)
Balance at Sep. 30, 2018	129,951		$ 12	322,468	(24)	(192,505)
Balance (in shares) at Sep. 30, 2018			11,942,729
Balance at Dec. 31, 2018	119,945		$ 12	323,279	(4)	(203,342)
Balance (in shares) at Dec. 31, 2018			11,954,528
Stock-based compensation expense	829			829
Issuance of common stock from stock grants and option exercises	106			106
Issuance of common stock from stock grants and option exercises, shares			19,576
Unrealized (loss) gain on available-for-sale securities	17				17
Net loss	(15,082)					(15,082)
Balance at Mar. 31, 2019	105,815		$ 12	324,214	13	(218,424)
Balance (in shares) at Mar. 31, 2019			11,974,104
Balance at Dec. 31, 2018	$ 119,945		$ 12	323,279	(4)	(203,342)
Balance (in shares) at Dec. 31, 2018			11,954,528
Issuance of common stock from option exercises, shares	41,219
Unrealized (loss) gain on available-for-sale securities	$ 35
Net loss	(41,568)
Balance at Sep. 30, 2019	81,272		$ 12	326,139	31	(244,910)
Balance (in shares) at Sep. 30, 2019			12,029,992
Balance at Mar. 31, 2019	105,815		$ 12	324,214	13	(218,424)
Balance (in shares) at Mar. 31, 2019			11,974,104
Stock-based compensation expense	903		$ 5,999	903
Issuance of common stock from option exercises	129			129
Issuance of common stock from option exercises, shares			28,425
Unrealized (loss) gain on available-for-sale securities	44				44
Net loss	(13,780)					(13,780)
Balance at Jun. 30, 2019	93,111		$ 12	325,246	57	(232,204)
Balance (in shares) at Jun. 30, 2019			12,008,528
Stock-based compensation expense	801		$ 7,393	801
Issuance of common stock from stock grants and option exercises	92			92
Issuance of common stock from stock grants and option exercises, shares			14,071
Unrealized (loss) gain on available-for-sale securities	(26)				(26)
Net loss	(12,706)					(12,706)
Balance at Sep. 30, 2019	$ 81,272		$ 12	$ 326,139	$ 31	$ (244,910)
Balance (in shares) at Sep. 30, 2019			12,029,992