Condensed Consolidated Statement of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Series A Redeemable Convertible Preferred Stock	Series B Redeemable Convertible Preferred Stock	Restricted Shares	Restricted stock units	Public Offering	At-the-Market Offering	Initial Public Offering	Common Stock	Common Stock Restricted Shares	Common Stock Restricted stock units	Common Stock Public Offering	Common Stock At-the-Market Offering	Common Stock Initial Public Offering	Additional Pain In Capital	Additional Pain In Capital Restricted Shares	Additional Pain In Capital Restricted stock units	Additional Pain In Capital Public Offering	Additional Pain In Capital At-the-Market Offering	Additional Pain In Capital Initial Public Offering	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)
Balance at Dec. 31, 2017	$ (31,929)								$ 1						$ 1,849						$ (33,779)	$ 0
Balance, shares at Dec. 31, 2017									4,562,640
Balance, temporary equity, shares at Dec. 31, 2017		15,527,951	4,792,716
Balance, temporary equity at Dec. 31, 2017		$ 41,674	$ 39,946
Conversion of convertible preferred stock into common stock upon the closing of initial public offering, Shares		(15,527,951)	(4,792,716)						15,870,559
Conversion of convertible preferred stock into common stock upon the closing of initial public offering	81,620	$ (41,674)	$ (39,946)						$ 1						81,619
Issuance of common stock								$ 89,370						$ 1						$ 89,369
Issuance of common stock, Shares														6,516,667
Vesting of shares, value				$ 865												$ 865
Vesting of shares										1,096,449
Stock-based compensation expense	316														316
Net loss	(9,859)																				(9,859)
Balance at Mar. 31, 2018	130,383								$ 3						174,018						(43,638)	0
Balance, shares at Mar. 31, 2018									28,046,315
Balance, temporary equity, shares at Mar. 31, 2018		0	0
Balance, temporary equity at Mar. 31, 2018		$ 0	$ 0
Balance at Dec. 31, 2017	(31,929)								$ 1						1,849						(33,779)	0
Balance, shares at Dec. 31, 2017									4,562,640
Balance, temporary equity, shares at Dec. 31, 2017		15,527,951	4,792,716
Balance, temporary equity at Dec. 31, 2017		$ 41,674	$ 39,946
Net loss	(23,450)
Unrealized gain (loss) on marketable securities	(30)
Balance at Jun. 30, 2018	117,164								$ 3						174,420						(57,229)	(30)
Balance, shares at Jun. 30, 2018									28,046,315
Balance, temporary equity, shares at Jun. 30, 2018		0	0
Balance, temporary equity at Jun. 30, 2018		$ 0	$ 0
Balance at Mar. 31, 2018	130,383								$ 3						174,018						(43,638)	0
Balance, shares at Mar. 31, 2018									28,046,315
Balance, temporary equity, shares at Mar. 31, 2018		0	0
Balance, temporary equity at Mar. 31, 2018		$ 0	$ 0
Stock-based compensation expense	402														402
Net loss	(13,591)																				(13,591)
Unrealized gain (loss) on marketable securities	(30)																					(30)
Balance at Jun. 30, 2018	117,164								$ 3						174,420						(57,229)	(30)
Balance, shares at Jun. 30, 2018									28,046,315
Balance, temporary equity, shares at Jun. 30, 2018		0	0
Balance, temporary equity at Jun. 30, 2018		$ 0	$ 0
Balance at Dec. 31, 2018	$ 104,204								$ 3						175,635						(71,393)	(41)
Balance, shares at Dec. 31, 2018	28,054,344								28,054,344
Issuance of common stock						$ 46,585						$ 1						$ 46,584
Issuance of common stock, Shares												7,200,000
Vesting of shares, value				$ 1												$ 1
Vesting of shares										500
Stock-based compensation expense	$ 662														662
Net loss	(11,069)																				(11,069)
Unrealized gain (loss) on marketable securities	73																					73
Balance at Mar. 31, 2019	140,455								$ 4						222,881						(82,462)	32
Balance, shares at Mar. 31, 2019									35,254,844
Balance at Dec. 31, 2018	$ 104,204								$ 3						175,635						(71,393)	(41)
Balance, shares at Dec. 31, 2018	28,054,344								28,054,344
Net loss	$ (29,401)
Unrealized gain (loss) on marketable securities	211
Balance at Jun. 30, 2019	$ 126,941								$ 4						227,561						(100,794)	170
Balance, shares at Jun. 30, 2019	35,817,393								35,817,393
Balance at Mar. 31, 2019	$ 140,455								$ 4						222,881						(82,462)	32
Balance, shares at Mar. 31, 2019									35,254,844
Issuance of common stock						$ 3,163	$ 582											$ 3,163	$ 582
Issuance of common stock, Shares												487,934	62,663
Vesting of shares, value					$ (15)												$ (15)
Vesting of shares										500	4,423
Exercise of stock options	6														6
Exercise of stock options, Shares									7,029
Stock-based compensation expense	944														944
Net loss	(18,332)																				(18,332)
Unrealized gain (loss) on marketable securities	138																					138
Balance at Jun. 30, 2019	$ 126,941								$ 4						$ 227,561						$ (100,794)	$ 170
Balance, shares at Jun. 30, 2019	35,817,393								35,817,393