Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Common Stock [Member] Private Placement [Member]	Common Stock [Member] Public Offering [Member]	Common Stock [Member] Private Investment in Public Equity (PIPE) [Member]	Common Stock [Member] Restricted Stock Units (RSUs) [Member]	Common Stock [Member]	Additional Paid-in Capital [Member] Private Placement [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]	Accumulated Deficit [Member]	Other Comprehensive Income (Loss) [Member]	Private Placement [Member]	Public Offering [Member]	Private Investment in Public Equity (PIPE) [Member]	Total
Shares, Outstanding					18,589,245
Issuance of common stock, value	$ 1					$ 74,322						$ 74,323
Balance, amount at Dec. 31, 2014					$ 2				$ 132,547	$ (71,867)					$ 60,682
Shares, Outstanding					25,964,886
Issuance of common stock, shares	7,242,992
Issuance of common stock under employee stock purchase plan ,amount									548						548
Issuance of common stock under employee stock purchase plan , shares					41,580
Exercise of stock options, amount									111						$ 111
Exercise of stock options, shares					77,784										77,784
Issuance of common stock for services, amount									194						$ 194
Issuance of common stock for services, shares					13,285										13,285
Stock-based compensation									2,634						$ 2,634
Other									30						30
Net loss										(29,621)					(29,621)
Balance, amount at Dec. 31, 2015					$ 3				210,386	(101,488)					108,901
Balance, shares at Dec. 31, 2015					25,964,886
Shares, Outstanding					25,964,886
Issuance of common stock, value		$ 1	$ 1				$ 80,836	$ 109,735					$ 80,837	$ 109,736
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
Exercise of stock options, amount									55						$ 55
Exercise of stock options, shares					25,134										25,134
Issuance of common stock for services, amount									187						$ 187
Issuance of common stock for services, shares					20,118										20,118
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
Balance, shares at Dec. 31, 2016															47,309,422
Issuance of common stock under employee stock purchase plan , shares					99,343
Exercise of stock options, amount									62						$ 62
Exercise of stock options, shares					35,759										35,759
Issuance of common stock for services, amount									201						$ 201
Issuance of common stock for services, shares					46,858										46,858
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