Consolidated Statements of Changes in Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	IPO	Secondary Public Offering [Member]	Series AA Redeemable Convertible Preferred Stock [Member]	Series AA Redeemable Convertible Preferred Stock [Member] IPO	Series X Redeemable Convertible Preferred Stock [Member]	Series X Redeemable Convertible Preferred Stock [Member] IPO	Common Stock [Member]	Common Stock [Member] IPO	Common Stock [Member] Secondary Public Offering [Member]	Common Stock [Member] Series AA Redeemable Convertible Preferred Stock [Member] IPO	Common Stock [Member] Series X Redeemable Convertible Preferred Stock [Member] IPO	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] IPO	Additional Paid-in Capital [Member] Secondary Public Offering [Member]	Additional Paid-in Capital [Member] Series AA Redeemable Convertible Preferred Stock [Member] IPO	Additional Paid-in Capital [Member] Series X Redeemable Convertible Preferred Stock [Member] IPO	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]
Beginning Balance at Dec. 31, 2014				$ 46,253		$ 548
Beginning balance, shares at Dec. 31, 2014				24,057,013		1,892,320
Beginning Balance at Dec. 31, 2014	$ (51,559)							$ 10					$ 76,472					$ (128,041)
Beginning balance, shares at Dec. 31, 2014								1,020,088
Stock-based compensation	2,380												2,380
Accretion of Series AA preferred stock to redemption value				$ 131
Accretion of Series AA preferred stock to redemption value	(131)												(131)
Issuance of common stock upon public offering		$ 36,495	$ 73,965						$ 70	$ 62				$ 36,425	$ 73,903
Issuance of common stock upon public offering, shares									6,966,333	6,210,000
Conversion of preferred stock into common stock, value					$ 46,384		$ 548				$ 75	$ 5				$ 46,309	$ 543
Conversion of preferred stock into common stock, value					$ (46,384)		$ (548)
Conversion of preferred stock into common stock, shares											7,536,331	466,319
Conversion of preferred stock into common stock, shares					(24,057,013)		(1,892,320)
Conversion of Convertible Bridge Notes into common stock upon initial public offering, value	2,027							$ 3					2,024
Conversion of Convertible Bridge Notes into common stock upon initial public offering, shares								337,932
Reclassification of fair value of warrant liability to equity upon initial public offering	215												215
Common stock issued pursuant to stock option plans, value	43												43
Common stock issued pursuant to stock option plans, shares								9,857
Common stock issued pursuant to employee stock purchase plan, value	63												63
Common stock issued pursuant to employee stock purchase plan, shares								13,143
Conversion of 2020 Convertible Notes into common stock, value	66,376							$ 39					66,337
Conversion of 2020 Convertible Notes into common stock, shares								3,863,391
Unrealized comprehensive loss on marketable securities	(11)																		$ (11)
Net loss	(67,982)																	(67,982)
Ending Balance at Dec. 31, 2015	108,813							$ 264					304,583					(196,023)	(11)
Ending balance, shares at Dec. 31, 2015								26,423,394
Stock-based compensation	2,909												2,909
Issuance of common stock upon public offering	3,997							$ 5					3,992
Issuance of common stock upon public offering, shares								482,689
Common stock issued pursuant to stock option plans, value	191							$ 1					190
Common stock issued pursuant to stock option plans, shares								54,310
Common stock issued pursuant to employee stock purchase plan, value	155												155
Common stock issued pursuant to employee stock purchase plan, shares								25,925
Unrealized comprehensive loss on marketable securities	(43)																		(43)
Net loss	(42,854)																	(42,854)
Ending Balance at Dec. 31, 2016	$ 73,168							$ 270					$ 311,829					$ (238,877)	$ (54)
Ending balance, shares at Dec. 31, 2016								26,986,318