Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Common Stock [Member] Public Offering [Member]	Common Stock [Member] Private Investment in Public Equity (PIPE) [Member]	Common Stock [Member] Underwriters' option	Common Stock [Member] Restricted Stock Units (RSUs) [Member]	Common Stock [Member]	Additional Paid-In Capital [Member] Public Offering [Member]	Additional Paid-In Capital [Member] Private Investment in Public Equity (PIPE) [Member]	Additional Paid-In Capital [Member] Underwriters' option	Additional Paid-In Capital [Member]	Accumulated Deficit [Member]	Other Comprehensive Income (Loss) [Member]	Public Offering [Member]	Private Investment in Public Equity (PIPE) [Member]	Underwriters' option	Total
Shares, Outstanding					25,964,886
Issuance of common stock, value	$ 1	$ 1				$ 80,836	$ 109,735					$ 80,837	$ 109,736
Balance, amount at Dec. 31, 2015					$ 3				$ 210,386	$ (101,488)					$ 108,901
Shares, Outstanding					47,309,422
Issuance of common stock, shares	8,625,000	12,600,230		5,000
Issuance of common stock under employee stock purchase plan, amount									576						576
Issuance of common stock under employee stock purchase plan, shares					69,054
Issuance of common stock for services, amount									187						$ 187
Issuance of common stock for services, shares					20,118										20,118
Exercise of stock options, amount									55						$ 55
Exercise of stock options, shares					25,134										25,134
Stock-based compensation									5,317						$ 5,317
Other comprehensive income (loss)											$ (71)				(71)
Net loss										(112,387)					(112,387)
Balance, amount at Dec. 31, 2016					$ 5				407,092	(213,875)	(71)				193,151
Balance, shares at Dec. 31, 2016					47,309,422
Shares, Outstanding					47,309,422
Shares, Outstanding					47,534,979
Issuance of common stock, shares				43,597
Issuance of common stock under employee stock purchase plan, amount									623						623
Issuance of common stock under employee stock purchase plan, shares					99,343
Issuance of common stock for services, amount									201						$ 201
Issuance of common stock for services, shares					46,858										46,858
Exercise of stock options, amount									62						$ 62
Exercise of stock options, shares					35,759										35,759
Stock-based compensation									9,590						$ 9,590
Other comprehensive income (loss)											24				24
Net loss										(64,339)					(64,339)
Balance, amount at Dec. 31, 2017					$ 5				417,568	(278,214)	(47)				139,312
Balance, shares at Dec. 31, 2017					47,534,979
Shares, Outstanding					47,534,979
Issuance of common stock, value			$ 1					$ 53,769						$ 53,770
Shares, Outstanding					62,516,627
Issuance of common stock, shares			14,375,000	410,506
Issuance of common stock under employee stock purchase plan, amount									491						$ 491
Balance, shares at Dec. 31, 2017															47,534,979
Issuance of common stock under employee stock purchase plan, shares					120,959
Issuance of common stock for services, amount									303						$ 303
Issuance of common stock for services, shares					75,183										75,183
Stock-based compensation									9,226						$ 9,226
Other comprehensive income (loss)											9				9
Adoption of ASU No. 2014-09 on January 1, 2018 (ASU 2014-09) at Dec. 31, 2018										4,000					4,000
Net loss										(91,298)					(91,298)
Balance, amount at Dec. 31, 2018					$ 6				$ 481,357	$ (365,512)	$ (38)				$ 115,813
Balance, shares at Dec. 31, 2018					62,516,627
Shares, Outstanding					62,516,627