CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS’ EQUITY - USD ($) $ in Thousands	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Total
Balance as of beginning of period at Dec. 31, 2017	$ 25	$ 224,025	$ (134,371)	$ 89,679
Balance as of beginning of period (in shares) at Dec. 31, 2017	24,538,309
Stockholders' Equity
Exercise of stock options		291		291
Exercise of stock options (in shares)	107,233
Stock-based compensation expense		6,483		6,483
Net loss			(41,519)	(41,519)
Balance as of end of period at Sep. 30, 2018	$ 25	230,799	(175,890)	54,934
Balance as of end of period (in shares) at Sep. 30, 2018	24,645,542
Balance as of beginning of period at Jun. 30, 2018	$ 25	228,355	(160,287)	68,093
Balance as of beginning of period (in shares) at Jun. 30, 2018	24,592,274
Stockholders' Equity
Exercise of stock options		113		113
Exercise of stock options (in shares)	53,268
Stock-based compensation expense		2,331		2,331
Net loss			(15,603)	(15,603)
Balance as of end of period at Sep. 30, 2018	$ 25	230,799	(175,890)	54,934
Balance as of end of period (in shares) at Sep. 30, 2018	24,645,542
Balance as of beginning of period at Dec. 31, 2018	$ 34	306,053	(201,109)	104,978
Balance as of beginning of period (in shares) at Dec. 31, 2018	33,863,077
Stockholders' Equity
At-the-market offering, net of sales agent commission	$ 1	2,129		2,130
At-the-market offering, net of sales agent commission (in shares)	532,304
Exercise of stock options		42		42
Exercise of stock options (in shares)	24,714
Issuance for employee stock purchase plan		545		545
Issuance for employee stock purchase plan (in shares)	123,664
Stock-based compensation expense		7,750		7,750
Net loss			(72,389)	(72,389)
Balance as of end of period at Sep. 30, 2019	$ 35	316,519	(273,498)	43,056
Balance as of end of period (in shares) at Sep. 30, 2019	34,543,759
Balance as of beginning of period at Jun. 30, 2019	$ 34	311,354	(250,322)	61,066
Balance as of beginning of period (in shares) at Jun. 30, 2019	33,882,967
Stockholders' Equity
At-the-market offering, net of sales agent commission	$ 1	2,129		2,130
At-the-market offering, net of sales agent commission (in shares)	532,304
Exercise of stock options		3		3
Exercise of stock options (in shares)	4,824
Issuance for employee stock purchase plan		545		545
Issuance for employee stock purchase plan (in shares)	123,664
Stock-based compensation expense		2,488		2,488
Net loss			(23,176)	(23,176)
Balance as of end of period at Sep. 30, 2019	$ 35	$ 316,519	$ (273,498)	$ 43,056
Balance as of end of period (in shares) at Sep. 30, 2019	34,543,759