CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Common stock	Additional Paid-in Capital	Accumulated Deficit	Total
Beginning balance at Dec. 31, 2018	$ 34	$ 430,824	$ (184,602)	$ 246,256
Beginning balance (in shares) at Dec. 31, 2018	34,410,725
Increase (Decrease) in Stockholders' Equity
Stock compensation expense		2,644		2,644
Issuance of common stock in connection with ESPP		295		295
Issuance of common stock in connection with ESPP (in shares)	12,105
Issuance of common stock in connection with exercise of stock options		54		54
Issuance of common stock in connection with exercise of stock options (in shares)	7,811
Change in unrealized gain (loss) on marketable securities		214		214
Net loss			(28,974)	(28,974)
Ending balance at Mar. 31, 2019	$ 34	434,031	(213,576)	220,489
Ending balance (in shares) at Mar. 31, 2019	34,430,641
Beginning balance at Dec. 31, 2019
Ending balance at Mar. 31, 2020
Beginning balance at Dec. 31, 2019	$ 44	606,307	(325,331)	281,020
Beginning balance (in shares) at Dec. 31, 2019	43,996,753
Increase (Decrease) in Stockholders' Equity
Stock compensation expense		5,475		5,475
Issuance of common stock in connection with ESPP		324		324
Issuance of common stock in connection with ESPP (in shares)	18,673
Issuance of common stock in connection with exercise of stock options		383		383
Issuance of common stock in connection with exercise of stock options (in shares)	72,964
Change in unrealized gain (loss) on marketable securities		63		63
Net loss			(34,164)	(34,164)
Ending balance at Mar. 31, 2020	$ 44	$ 612,552	$ (359,495)	$ 253,101
Ending balance (in shares) at Mar. 31, 2020	44,088,390