Consolidated Statements of Stockholders’ Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2014		17,551,375
Beginning balance at Dec. 31, 2014	$ 147,074	$ 22	$ 157,581	$ (180)	$ (10,349)
Increase (decrease) in shareholders' equity
Exercise of stock options (in shares)		409,683
Exercise of stock options	761	$ 0	761
Vesting of common stock	112		112
Issuance of common stock in conjunction with Controlled Equity Offering, net of commissions and issuance costs (in shares)		509,741
Issuance of common stock in conjunction with Controlled Equity Offering, net of commissions and issuance costs	9,657	$ 1	9,656
Issuance of common stock in conjunction with warrant exercises (in shares)		3,484
Net unrealized gain (loss) on available-for-sale securities	22			22
Stock issued under employee stock purchase plan (in shares)		31,179
Stock issued under employee stock purchase plan	407		407
Stock-based compensation	9,956		9,956
Net loss	(51,802)				(51,802)
Ending balance (in shares) at Dec. 31, 2015		18,505,462
Ending balance at Dec. 31, 2015	116,187	$ 23	178,473	(158)	(62,151)
Increase (decrease) in shareholders' equity
Exercise of stock options (in shares)		586,956
Exercise of stock options	3,042	$ 1	3,041
Vesting of common stock	34		34
Issuance of common stock in conjunction with Secondary Offering, net of commissions and issuance costs (in shares)		2,875,000
Issuance of common stock in conjunction with Secondary Offering, net of commissions and issuance costs	61,822	$ 3	61,819
Net unrealized gain (loss) on available-for-sale securities	(35)			(35)
Stock issued under employee stock purchase plan (in shares)		46,226
Stock issued under employee stock purchase plan	620		620
Stock-based compensation	10,571		10,571
Net loss	(60,058)				(60,058)
Ending balance (in shares) at Dec. 31, 2016		22,013,644
Ending balance at Dec. 31, 2016	132,183	$ 27	254,558	(193)	(122,209)
Increase (decrease) in shareholders' equity
Exercise of stock options (in shares)		1,183,353
Exercise of stock options	$ 9,034	$ 1	9,033
Restricted stock units vested (in shares)	64,471
Restricted stock units vested	$ 201
Net unrealized gain (loss) on available-for-sale securities	26			26
Stock issued under employee stock purchase plan (in shares)		59,083
Stock issued under employee stock purchase plan	766		766
Stock-based compensation	13,406		13,406
Net loss	(89,490)				(89,490)
Ending balance (in shares) at Dec. 31, 2017		23,320,551
Ending balance at Dec. 31, 2017	$ 66,126	$ 28	$ 277,964	$ (167)	$ (211,699)