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, 2014		18,272
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
Issuance of common stock net of issuance costs	12,518	$ 0	12,518
Issuance of common stock warrants	0
Issuance of common stock for employee stock purchase plan (in shares)		136
Issuance of common stock for employee stock purchase plan	1,295		1,295
Exercise of stock options (in shares)		202
Exercise of stock options	876		876
Exercise of common stock warrant, net (in shares)		0
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
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)		1,333
Issuance of common stock net of issuance costs	26,073		26,073
Issuance of common stock warrants	0
Issuance of common stock for employee stock purchase plan (in shares)		139
Issuance of common stock for employee stock purchase plan	1,489		1,489
Exercise of stock options (in shares)		349
Exercise of stock options	2,607		2,607
Exercise of common stock warrant, net (in shares)		133
Exercise of common stock warrants, net	0		0
Vesting of restricted stock units (in shares)		5
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		21,529
Balances at at Dec. 31, 2016	12,305	$ 2	281,900	(57)	(269,540)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock net of issuance costs (in shares)		3,450
Issuance of common stock net of issuance costs	56,486		56,486
Issuance of common stock warrants	674		674
Issuance of common stock for employee stock purchase plan (in shares)		139
Issuance of common stock for employee stock purchase plan	$ 1,793		1,793
Exercise of stock options (in shares)	227,696	228
Exercise of stock options	$ 1,086		1,086
Exercise of common stock warrant, net (in shares)		29
Exercise of common stock warrants, net	0
Vesting of restricted stock units (in shares)		46
Stock-based compensation	11,369		11,369
Net loss	(43,562)
Other comprehensive loss	(42)			(42)
Balance at (in shares) at Dec. 31, 2017		25,421
Balances at at Dec. 31, 2017	$ 40,109	$ 2	$ 353,308	$ (99)	$ (313,102)