CONSOLIDATED STATEMENTS OF REDEEMABLE CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Series A Redeemable Convertible Preferred Stock	Series B Redeemable Convertible Preferred Stock	Series B‑1 Tranche A Redeemable Convertible Preferred Stock	Series B‑1 Tranche B Redeemable Convertible Preferred Stock	Common Stock	Additional Paid-in Capital ASU 2018-07	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings ASU 2018-07	Retained Earnings	Total
Decrease in Temporary Equity and increase (decrease) in Stockholders' Equity
ASU 2018‑07 modified retrospective adjustment						$ (13)			$ 13
Balance at Dec. 31, 2017	$ 23,866	$ 24,550	$ 24,423	$ 31,874
Balance (in shares) at Dec. 31, 2017	33,499,900	25,000,000	42,372,882	54,067,796
Balance at Dec. 31, 2017					$ 3		$ 1,377			$ (57,170)	$ (55,790)
Balance (in shares) at Dec. 31, 2017					12,639
Decrease in Temporary Equity and increase (decrease) in Stockholders' Equity
Stock-based compensation expense							806				806
Reorganization adjustment					$ (3)		3
Conversion of preferred stock to common stock upon initial public offering	$ (23,866)	$ (24,550)	$ (24,423)	$ (31,874)	$ 8		104,705				104,713
Conversion of preferred stock into common stock upon initial public offering (in shares)	(33,499,900)	(25,000,000)	(42,372,882)	(54,067,796)	8,084,414
Issuance of common stock upon initial public offering, net of issuance costs of $9,376					$ 5		65,619				65,624
Issuance of common stock upon initial public offering, net of issuance costs of $9,376 (in shares)					5,000,000
Exercise of stock options							3				3
Exercise of stock options (in shares)					908
Net loss										(24,873)	(24,873)
Balance at Sep. 30, 2018					$ 13		172,500			(82,030)	90,483
Balance (in shares) at Sep. 30, 2018					13,097,961
Balance at Jun. 30, 2018	$ 23,866	$ 24,550	$ 24,423	$ 31,874
Balance (in shares) at Jun. 30, 2018	33,499,900	25,000,000	42,372,882	54,067,796
Balance at Jun. 30, 2018							1,840			(73,557)	(71,717)
Balance (in shares) at Jun. 30, 2018					12,639
Decrease in Temporary Equity and increase (decrease) in Stockholders' Equity
Stock-based compensation expense							333				333
Conversion of preferred stock to common stock upon initial public offering	$ (23,866)	$ (24,550)	$ (24,423)	$ (31,874)	$ 8		104,705				104,713
Conversion of preferred stock into common stock upon initial public offering (in shares)	(33,499,900)	(25,000,000)	(42,372,882)	(54,067,796)	8,084,414
Issuance of common stock upon initial public offering, net of issuance costs of $9,376					$ 5		65,619				65,624
Issuance of common stock upon initial public offering, net of issuance costs of $9,376 (in shares)					5,000,000
Exercise of stock options							3				3
Exercise of stock options (in shares)					908
Net loss										(8,473)	(8,473)
Balance at Sep. 30, 2018					$ 13		172,500			(82,030)	90,483
Balance (in shares) at Sep. 30, 2018					13,097,961
Balance at Dec. 31, 2018					$ 13		172,988	$ (9)		(90,109)	82,883
Balance (in shares) at Dec. 31, 2018					13,124,842
Decrease in Temporary Equity and increase (decrease) in Stockholders' Equity
Stock-based compensation expense							1,748				1,748
Exercise of stock options							40				40
Exercise of stock options (in shares)					9,696
Unrealized gain on investments held								31			31
Net loss										(29,808)	(29,808)
Balance at Sep. 30, 2019					$ 13		174,776	22		(119,917)	54,894
Balance (in shares) at Sep. 30, 2019					13,134,538
Balance at Jun. 30, 2019					$ 13		174,205	54		(116,432)	57,840
Balance (in shares) at Jun. 30, 2019					13,134,538
Decrease in Temporary Equity and increase (decrease) in Stockholders' Equity
Stock-based compensation expense							571				571
Unrealized gain on investments held								(32)			(32)
Net loss										(3,485)	(3,485)
Balance at Sep. 30, 2019					$ 13		$ 174,776	$ 22		$ (119,917)	$ 54,894
Balance (in shares) at Sep. 30, 2019					13,134,538