Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common stock	Treasury stock	Additional paid-in capital	Accumulated deficit
Balance, beginning of year at Dec. 31, 2014	$ 95,639	$ 150	$ (260)	$ 229,795	$ (134,046)
Balance, beginning of year (in shares) at Dec. 31, 2014		14,985,704	72,727
Proceeds from follow-on offering, net of costs	61,397	$ 30		61,367
Proceeds from follow-on offering, net of costs (in shares)		3,000,000
Employee stock-based compensation expense	2,382			2,382
Stock option exercises	119			119
Stock option exercises (in shares)		36,189
Employee stock purchase program (ESPP)	564			564
Employee stock purchase program (ESPP) (in shares)		44,250
Net loss	(41,230)				(41,230)
Balance, end of year at Dec. 31, 2015	118,871	$ 180	$ (260)	294,227	(175,276)
Balance, end of year (in shares) at Dec. 31, 2015		18,066,143	72,727
Employee stock-based compensation expense	3,236			3,236
Stock option exercises	923	$ 5		918
Stock option exercises (in shares)		478,099
Employee stock purchase program (ESPP)	753	$ 1		752
Employee stock purchase program (ESPP) (in shares)		122,667
Vested restricted stock (in shares)		4,500
Net loss	(40,166)				(40,166)
Balance, end of year at Dec. 31, 2016	83,617	$ 186	$ (260)	299,133	(215,442)
Balance, end of year (in shares) at Dec. 31, 2016		18,671,409	72,727
Employee stock-based compensation expense	6,766			6,766
Stock option exercises	1,346	$ 5		1,341
Stock option exercises (in shares)		480,236
Employee stock purchase program (ESPP)	647	$ 1		646
Employee stock purchase program (ESPP) (in shares)		108,081
Vested restricted stock		$ 3		(3)
Vested restricted stock (in shares)		293,910
Shares withheld for tax obligations on vested RSUs	(725)	$ (1)		(724)
Shares withheld for tax obligations on vested RSUs, shares		(78,934)
Net loss	(64,028)				(64,028)
Balance, end of year at Dec. 31, 2017	$ 27,623	$ 194	$ (260)	$ 307,159	$ (279,470)
Balance, end of year (in shares) at Dec. 31, 2017		19,474,702	72,727