Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Convertible preferred stock	Common stock	Treasury stock	Additional paid-in capital	Accumulated deficit	Total
Balance, beginning of year at Dec. 31, 2013	$ 150,456
Balance, beginning of year (in shares) at Dec. 31, 2013	24,593,087
Changes in Temporary Equity
Conversion of convertible preferred stock to common stock	$ (150,456)
Conversion of convertible preferred stock to common stock (in shares)	(24,593,087)
Balance, beginning of year at Dec. 31, 2013		$ 3	$ (260)	$ 1,819	$ (128,235)	$ (126,673)
Balance, beginning of year (in shares) at Dec. 31, 2013		279,879	72,727
Changes in Stockholders' Equity
Conversion of convertible preferred stock to common stock		$ 89		150,367		150,456
Conversion of convertible preferred stock to common stock (in shares)		8,942,925
Proceeds from IPO/follow-on offering, net of costs		$ 58		76,977		77,035
Proceeds from IPO/follow-on offering, net of costs (in shares)		5,750,000
Employee stock-based compensation expense				594		594
Stock option exercises				38		38
Stock option exercises (in shares)		12,900
Net loss					(5,811)	(5,811)
Balance, end of year at Dec. 31, 2014		$ 150	$ (260)	229,795	(134,046)	95,639
Balance, end of year (in shares) at Dec. 31, 2014		14,985,704	72,727
Changes in Stockholders' Equity
Proceeds from IPO/follow-on offering, net of costs		$ 30		61,367		61,397
Proceeds from IPO/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		$ 180	$ (260)	294,227	(175,276)	118,871
Balance, end of year (in shares) at Dec. 31, 2015		18,066,143	72,727
Changes in Stockholders' Equity
Employee stock-based compensation expense				3,236		3,236
Stock option exercises		$ 5		918		923
Stock option exercises (in shares)		478,099
Employee stock purchase program (ESPP)		$ 1		752		753
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		$ 186	$ (260)	$ 299,133	$ (215,442)	$ 83,617
Balance, end of year (in shares) at Dec. 31, 2016		18,671,409	72,727