Condensed Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Open Market Sale Agreement	Common Stock	Common Stock Open Market Sale Agreement	Additional Paid In Capital	Additional Paid In Capital Open Market Sale Agreement	Accumulated Deficit
Balance at Dec. 31, 2018	$ 155,007		$ 37		$ 220,240		$ (65,270)
Balance, Shares at Dec. 31, 2018			36,760,536
Issuance of common stock upon exercise of stock options	25				25
Issuance of common stock upon exercise of stock options, Shares			50,533
Vesting of restricted stock and early exercised options	38				38
Stock-based compensation expense	964				964
Net loss and comprehensive loss	(11,733)						(11,733)
Balance at Mar. 31, 2019	144,301		$ 37		221,267		(77,003)
Balance, Shares at Mar. 31, 2019			36,811,069
Balance at Dec. 31, 2018	155,007		$ 37		220,240		(65,270)
Balance, Shares at Dec. 31, 2018			36,760,536
Net loss and comprehensive loss	(25,786)
Balance at Jun. 30, 2019	131,846		$ 37		222,865		(91,056)
Balance, Shares at Jun. 30, 2019			36,871,175
Balance at Mar. 31, 2019	144,301		$ 37		221,267		(77,003)
Balance, Shares at Mar. 31, 2019			36,811,069
Issuance of common stock upon exercise of stock options	87				87
Issuance of common stock upon exercise of stock options, Shares			34,480
Issuance of common stock under employee stock plans	308				308
Issuance of common stock under employee stock plans, Shares			25,626
Vesting of restricted stock and early exercised options	37				37
Stock-based compensation expense	1,166				1,166
Net loss and comprehensive loss	(14,053)						(14,053)
Balance at Jun. 30, 2019	131,846		$ 37		222,865		(91,056)
Balance, Shares at Jun. 30, 2019			36,871,175
Balance at Dec. 31, 2019	171,427		$ 38		274,494		(103,105)
Balance, Shares at Dec. 31, 2019			38,040,693
Issuance of common stock upon exercise of stock options	192				192
Issuance of common stock upon exercise of stock options, Shares			39,393
Issuance of common stock in at-the-market offering, net offering		$ 24,094		$ 1		$ 24,093
Issuance of common stock in at-the-market offering, net offering, Shares				448,755
Vesting of restricted stock and early exercised options	36				36
Stock-based compensation expense	1,927				1,927
Net loss and comprehensive loss	(22,824)						(22,824)
Balance at Mar. 31, 2020	174,852		$ 39		300,742		(125,929)
Balance, Shares at Mar. 31, 2020			38,528,841
Balance at Dec. 31, 2019	$ 171,427		$ 38		274,494		(103,105)
Balance, Shares at Dec. 31, 2019			38,040,693
Issuance of common stock upon exercise of stock options, Shares	63,171
Net loss and comprehensive loss	$ (51,673)
Balance at Jun. 30, 2020	149,311		$ 39		304,050		(154,778)
Balance, Shares at Jun. 30, 2020			38,561,026
Balance at Mar. 31, 2020	174,852		$ 39		300,742		(125,929)
Balance, Shares at Mar. 31, 2020			38,528,841
Issuance of common stock upon exercise of stock options	204				204
Issuance of common stock upon exercise of stock options, Shares			23,778
Issuance of common stock under employee stock plans	350				350
Issuance of common stock under employee stock plans, Shares			8,407
Vesting of restricted stock and early exercised options	36				36
Stock-based compensation expense	2,718				2,718
Net loss and comprehensive loss	(28,849)						(28,849)
Balance at Jun. 30, 2020	$ 149,311		$ 39		$ 304,050		$ (154,778)
Balance, Shares at Jun. 30, 2020			38,561,026