Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Other Comprehensive Loss	Accumulated Deficit
Balance at (in shares) at Dec. 31, 2013		14,619,818
Balances at at Dec. 31, 2013	$ 31,469	$ 1	$ 158,278	$ 22	$ (126,832)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock net of issuance costs (in shares)		3,318,917
Issuance of common stock net of issuance costs	56,987	$ 1	56,986
Issuance of common stock for employee stock purchase plan (in shares)		141,386
Issuance of common stock for employee stock purchase plan	987		987
Exercise of stock options (in shares)		164,394
Exercise of stock options	411		411
Exercise of common stock warrant, net (in shares)		27,269
Exercise of common stock warrants, net	136		136
Stock-based compensation	4,926		4,926
Net loss	(50,038)				(50,038)
Other comprehensive loss	(65)			(65)
Balance at (in shares) at Dec. 31, 2014		18,271,784
Balances at at Dec. 31, 2014	44,813	$ 2	221,724	(43)	(176,870)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock net of issuance costs (in shares)		960,400
Issuance of common stock net of issuance costs	12,518		12,518
Issuance of common stock for employee stock purchase plan (in shares)		136,078
Issuance of common stock for employee stock purchase plan	1,295		1,295
Exercise of stock options (in shares)		201,622
Exercise of stock options	876		876
Exercise of common stock warrant, net (in shares)		250
Exercise of common stock warrants, net	2		2
Stock-based compensation	6,278		6,278
Net loss	(45,581)				(45,581)
Other comprehensive loss	14			14
Balance at (in shares) at Dec. 31, 2015		19,570,134
Balances at at Dec. 31, 2015	20,215	$ 2	242,693	(29)	(222,451)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock net of issuance costs (in shares)		1,331,539
Issuance of common stock net of issuance costs	26,073		26,073
Issuance of common stock for employee stock purchase plan (in shares)		139,195
Issuance of common stock for employee stock purchase plan	$ 1,489		1,489
Exercise of stock options (in shares)	348,983	348,983
Exercise of stock options	$ 2,607		2,607
Exercise of common stock warrant, net (in shares)		132,757
Exercise of common stock warrants, net	0
Vesting of restricted stock units (in shares)		5,000
Stock-based compensation	9,038		9,038
Net loss	(47,089)				(47,089)
Other comprehensive loss	(28)			(28)
Balance at (in shares) at Dec. 31, 2016		21,527,608
Balances at at Dec. 31, 2016	$ 12,305	$ 2	$ 281,900	$ (57)	$ (269,540)