Consolidated Statements of Changes in Convertible Preferred Shares/Stock and Members'/Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Series A Convertible Preferred Shares	Series A Convertible Preferred Stock	Series B Convertible Preferred Stock	Common Stock	Common Stock Class A-1 Common	Common Stock Class A Common	Common Stock Class B Common	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Loss
Balances at Dec. 31, 2014	$ (11,473)					$ 277	$ 387	$ 147		$ (12,284)
Balances (in shares) at Dec. 31, 2014						165,000	335,000	355,000
Temporary Equity Balances at Dec. 31, 2014		$ 13,345
Temporary Equity Balances (in shares) at Dec. 31, 2014		2,173,000
Forfeiture of Common B shares								(1,000)
Share-based compensation	20							$ 20
Conversion from LLC to corporation		$ (13,345)	$ 13,345			$ (277)	$ (387)	$ (167)	$ 831
Conversion from LLC to corporation (in shares)		(2,173,000)	2,173,000		753,000	(165,000)	(335,000)	(354,000)
Deemed dividend to convertible preferred stockholders	(228)		$ 228						(228)
Issuance of convertible preferred shares for cash, net in issuance costs				$ 43,679
Issuance of convertible preferred shares for cash, net in issuance costs (in shares)				4,930,000
Issuance of convertible preferred shares for research and development services				$ 1,059
Issuance of convertible preferred shares for research and development services (in shares)				70,000
Stock-based compensation after conversion from an LLC to corporation	747								747
Issuance of common stock upon exercise of stock options	23								23
Issuance of common stock upon exercise of stock options (in shares)					4,000
Unrealized loss on marketable securities	(1)										$ (1)
Net loss	(11,295)									(11,295)
Balances at Dec. 31, 2015	(22,207)								1,373	(23,579)	(1)
Balances (in shares) at Dec. 31, 2015					757,000
Temporary Equity Balances at Dec. 31, 2015			$ 13,573	$ 44,738
Temporary Equity Balances (in shares) at Dec. 31, 2015			2,173,000	5,000,000
Conversion of preferred stock to common stock upon initial public offering	58,311		$ (13,573)	$ (44,738)	$ 1				58,310
Conversion of preferred stock to common stock upon initial public offering (in shares)			(2,173,000)	(5,000,000)	7,173,000
Issuance of common stock in connection with initial public offering/ follow-on offering, net of offering costs	47,266								47,266
Issuance of common stock in connection with initial public offering/ follow-on offering, net of offering costs (in shares)					5,482,000
Issuance of common stock in connection with employee stock purchase plan	76								76
Issuance of common stock in connection with employee stock purchase plan (in shares)					19,000
Stock-based compensation after conversion from an LLC to corporation	1,221								1,221
Unrealized loss on marketable securities	(3)										(3)
Net loss	(21,698)									(21,698)
Balances at Dec. 31, 2016	62,966				$ 1				108,246	(45,277)	(4)
Balances (in shares) at Dec. 31, 2016					13,431,000
Issuance of common stock in connection with initial public offering/ follow-on offering, net of offering costs	11,380				$ 1				11,379
Issuance of common stock in connection with initial public offering/ follow-on offering, net of offering costs (in shares)					3,000,000
Issuance of common stock in connection with employee stock purchase plan	131								131
Issuance of common stock in connection with employee stock purchase plan (in shares)					39,000
Stock-based compensation after conversion from an LLC to corporation	2,492								2,492
Issuance of common stock upon exercise of stock options	$ 702								702
Issuance of common stock upon exercise of stock options (in shares)	200,600				200,000
Unrealized loss on marketable securities	$ (98)										(98)
Net loss	(27,236)									(27,236)
Balances at Dec. 31, 2017	$ 50,337				$ 2				$ 122,950	$ (72,513)	$ (102)
Balances (in shares) at Dec. 31, 2017					16,670,000