Condensed Statements of Stockholders' (Deficit) Equity - USD ($) shares in Thousands, $ in Thousands	Common stock	Additional paid-in capital	Accumulated deficit	Total
Stockholders' (deficit) equity beginning balance at Dec. 31, 2017	$ 53	$ 12,486	$ (52,257)	$ (39,718)
Stockholders' (deficit) equity shares beginning balance at Dec. 31, 2017	5,342
Stockholders' (Deficit) Equity Rollforward
Issuance of common stock upon vesting of restricted stock	$ 2	(2)
Issuance of common stock upon vesting of restricted stock (in shares)	220
Stock-based compensation		1,984		1,984
Conversion of preferred stock upon IPO	$ 203	132,183		132,386
Conversion of preferred stock upon IPO (in shares)	20,306
Issuance of common stock upon IPO, net of underwriters' discount and other offering costs of $2,902	$ 74	106,431		106,505
Issuance of common stock upon IPO, net of underwriters' discount and other offering costs of $2,902 (in shares)	7,360
Issuance of common stock upon exercise of stock options		2		2
Issuance of common stock upon exercise of stock options (in shares)	1
Net loss			(39,188)	(39,188)
Stockholders' (deficit) equity shares ending balance at Sep. 30, 2018	33,229
Stockholders' (deficit) equity ending balance at Sep. 30, 2018	$ 332	253,084	(91,445)	161,971
Stockholders' (deficit) equity beginning balance at Jun. 30, 2018	$ 332	252,473	(77,264)	175,541
Stockholders' (deficit) equity shares beginning balance at Jun. 30, 2018	33,155
Stockholders' (Deficit) Equity Rollforward
Issuance of common stock upon vesting of restricted stock (in shares)	73
Stock-based compensation		720		720
Issuance of common stock upon IPO, net of underwriters' discount and other offering costs of $2,902		(111)		(111)
Issuance of common stock upon exercise of stock options		2		2
Issuance of common stock upon exercise of stock options (in shares)	1
Net loss			(14,181)	(14,181)
Stockholders' (deficit) equity shares ending balance at Sep. 30, 2018	33,229
Stockholders' (deficit) equity ending balance at Sep. 30, 2018	$ 332	253,084	(91,445)	161,971
Stockholders' (deficit) equity beginning balance at Dec. 31, 2018	$ 333	254,516	(105,538)	$ 149,311
Stockholders' (deficit) equity shares beginning balance at Dec. 31, 2018	33,341			33,341
Stockholders' (Deficit) Equity Rollforward
Issuance of common stock upon vesting of restricted stock	$ 2	(2)
Issuance of common stock upon vesting of restricted stock (in shares)	193
Stock-based compensation		6,966		$ 6,966
Issuance of common stock upon exercise of stock options	$ 2	280		282
Issuance of common stock upon exercise of stock options (in shares)	142
Net loss			(43,632)	$ (43,632)
Stockholders' (deficit) equity shares ending balance at Sep. 30, 2019	33,676			33,676
Stockholders' (deficit) equity ending balance at Sep. 30, 2019	$ 337	261,760	(149,170)	$ 112,927
Stockholders' (deficit) equity beginning balance at Jun. 30, 2019	$ 336	259,090	(134,380)	125,046
Stockholders' (deficit) equity shares beginning balance at Jun. 30, 2019	33,608
Stockholders' (Deficit) Equity Rollforward
Issuance of common stock upon vesting of restricted stock	$ 1	(1)
Issuance of common stock upon vesting of restricted stock (in shares)	54
Stock-based compensation		2,641		2,641
Issuance of common stock upon exercise of stock options		30		30
Issuance of common stock upon exercise of stock options (in shares)	14
Net loss			(14,790)	$ (14,790)
Stockholders' (deficit) equity shares ending balance at Sep. 30, 2019	33,676			33,676
Stockholders' (deficit) equity ending balance at Sep. 30, 2019	$ 337	$ 261,760	$ (149,170)	$ 112,927