CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Total
Balance at beginning at Dec. 31, 2017	$ 4	$ 4,001	$ (2)	$ (57,525)	$ (53,522)
Balance at beginning (in shares) at Dec. 31, 2017	3,970,586
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Unrealized gain (loss) on marketable securities			1		1
Equity-based compensation expense		482			482
Net loss				(8,892)	(8,892)
Balance at end at Mar. 31, 2018	$ 4	4,483	(1)	(66,417)	(61,931)
Balance at end (in shares) at Mar. 31, 2018	3,970,586
Balance at beginning at Dec. 31, 2017	$ 4	4,001	(2)	(57,525)	(53,522)
Balance at beginning (in shares) at Dec. 31, 2017	3,970,586
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Unrealized gain (loss) on marketable securities					2
Net loss					(34,360)
Balance at end at Sep. 30, 2018	$ 25	194,204		(91,886)	102,343
Balance at end (in shares) at Sep. 30, 2018	25,241,766
Balance at beginning at Mar. 31, 2018	$ 4	4,483	(1)	(66,417)	(61,931)
Balance at beginning (in shares) at Mar. 31, 2018	3,970,586
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Unrealized gain (loss) on marketable securities			1		1
Restricted shares forfeited during the period (in shares)	(852)
Equity-based compensation expense		1,089			1,089
Reclassification of warrant to stockholders' equity		93			93
Conversion of convertible preferred stock into common stock	$ 15	109,217			109,232
Conversion of convertible preferred stock into common stock (in shares)	15,109,950
Sale of common shares sold in IPO, net of offering costs	$ 6	77,829			77,835
Sale of common shares sold in IPO, net of offering costs (in shares)	6,164,000
Net loss				(14,707)	(14,707)
Balance at end at Jun. 30, 2018	$ 25	192,711		(81,124)	111,612
Balance at end (in shares) at Jun. 30, 2018	25,243,684
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Restricted shares forfeited during the period (in shares)	(2,367)
Exercise of stock options		3			3
Exercise of stock options (in shares)	449
Equity-based compensation expense		1,486			1,486
Sale of common shares sold in IPO, net of offering costs		4			4
Net loss				(10,762)	(10,762)
Balance at end at Sep. 30, 2018	$ 25	194,204		(91,886)	102,343
Balance at end (in shares) at Sep. 30, 2018	25,241,766
Balance at beginning at Dec. 31, 2018	$ 26	213,453	(8)	(106,848)	106,623
Balance at beginning (in shares) at Dec. 31, 2018	26,217,701
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Unrealized gain (loss) on marketable securities			24		24
Restricted shares forfeited during the period (in shares)	(2,237)
Exercise of stock options		13			13
Exercise of stock options (in shares)	1,983
Equity-based compensation expense		1,618			1,618
Net loss				(10,755)	(10,755)
Balance at end at Mar. 31, 2019	$ 26	215,084	16	(117,603)	97,523
Balance at end (in shares) at Mar. 31, 2019	26,217,447
Balance at beginning at Dec. 31, 2018	$ 26	213,453	(8)	(106,848)	106,623
Balance at beginning (in shares) at Dec. 31, 2018	26,217,701
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Unrealized gain (loss) on marketable securities					34
Net loss					(39,427)
Balance at end at Sep. 30, 2019	$ 30	268,009	26	(146,275)	121,790
Balance at end (in shares) at Sep. 30, 2019	29,678,422
Balance at beginning at Mar. 31, 2019	$ 26	215,084	16	(117,603)	97,523
Balance at beginning (in shares) at Mar. 31, 2019	26,217,447
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Unrealized gain (loss) on marketable securities			42		42
Exercise of stock options		79			79
Exercise of stock options (in shares)	12,883
Equity-based compensation expense		1,814			1,814
Sale of common shares	$ 3	42,019			42,022
Sale of common shares (in shares)	3,000,000
Net loss				(12,525)	(12,525)
Balance at end at Jun. 30, 2019	$ 29	258,996	58	(130,128)	128,955
Balance at end (in shares) at Jun. 30, 2019	29,230,330
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Unrealized gain (loss) on marketable securities			(32)		(32)
Restricted shares forfeited during the period (in shares)	(1,973)
Exercise of stock options (in shares)	65
Equity-based compensation expense		2,688			2,688
Sale of common shares	$ 1	6,325			6,326
Sale of common shares (in shares)	450,000
Net loss				(16,147)	(16,147)
Balance at end at Sep. 30, 2019	$ 30	$ 268,009	$ 26	$ (146,275)	$ 121,790
Balance at end (in shares) at Sep. 30, 2019	29,678,422