Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($)	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive (Gain) Loss	Accumulated Deficit	Redeemable convertible preferred stock	Total
Balance at Dec. 31, 2014					$ 20,576,000
Balance (in shares) at Dec. 31, 2014					42,037,500
Increase (Decrease) in Redeemable Convertible Preferred Stock
Issuance of redeemable convertible preferred stock					$ 16,345,000
Issuance of Series C redeemable convertible preferred stock (in shares)					35,147,617
Accretion of redemption of convertible preferred stock to redemption value					$ 75,000	$ 75,000
Balance at Dec. 31, 2015					$ 36,996,000
Balance (in shares) at Dec. 31, 2015					77,185,117
Balance at Dec. 31, 2014		$ 37,000	$ (100,000)	$ (12,558,000)		(12,621,000)
Balance (in shares) at Dec. 31, 2014	228,557
Increase (Decrease) in Stockholders' Equity (Deficit)
Accretion of redeemable convertible preferred stock		(75,000)				(75,000)
Stock-based compensation expense		99,000				99,000
Issuance of common stock upon the exercise of options		57,000				57,000
Issuance of common stock upon the exercise of options (in shares)	43,852
Other comprehensive gain (loss)			(2,000)			(2,000)
Net loss				(14,858,000)		(14,858,000)
Balance at Dec. 31, 2015		118,000	(102,000)	(27,416,000)		(27,400,000)
Balance (in shares) at Dec. 31, 2015	272,409
Increase (Decrease) in Redeemable Convertible Preferred Stock
Issuance of redeemable convertible preferred stock					$ 22,488,000
Issuance of Series C redeemable convertible preferred stock (in shares)					45,189,794
Settlement of fair value of redeemable convertible preferred stock tranche liability					$ 5,837,000
Exercise of redeemable convertible preferred stock warrant liability					$ 1,025,000
Exercise of redeemable convertible preferred stock warrant liability (in shares)					1,999,998
Accretion of redemption of convertible preferred stock to redemption value					$ 558,000	558,000
Conversion of redeemable convertible preferred stock to common stock at closing of initial public offering					$ (66,904,000)
Conversion of redeemable convertible preferred stock to common stock at closing of initial public offering (in shares)					(124,374,909)
Balance at Dec. 31, 2016						36,996,000
Balance (in shares) at Dec. 31, 2016					0
Increase (Decrease) in Stockholders' Equity (Deficit)
Conversion of redeemable convertible preferred stock to common stock at closing of initial public offering		66,904,000				66,904,000
Conversion of redeemable convertible preferred stock to common stock at closing of initial public offering (in shares)	8,577,571
Issuance of common stock upon public offering (2017); initial public offering (2016), net of issuance costs		83,648,000				83,648,000
Issuance of common stock upon public offering (2017); initial public offering (2016) (in shares)	7,752,972
Accretion of redeemable convertible preferred stock		(558,000)				(558,000)
Stock-based compensation expense		2,130,000				2,130,000
Issuance of common stock upon the exercise of options		151,000				151,000
Issuance of common stock upon the exercise of options (in shares)	119,328
Other comprehensive gain (loss)			(143,000)			(143,000)
Net loss				(37,177,000)		(37,177,000)
Balance at Dec. 31, 2016		152,393,000	(245,000)	(64,593,000)		$ 87,555,000
Balance (in shares) at Dec. 31, 2016	16,722,280					16,722,280
Increase (Decrease) in Redeemable Convertible Preferred Stock
Accretion of redemption of convertible preferred stock to redemption value						$ 0
Balance (in shares) at Dec. 31, 2017					0
Increase (Decrease) in Stockholders' Equity (Deficit)
Issuance of common stock upon public offering (2017); initial public offering (2016), net of issuance costs		64,547,000				64,547,000
Issuance of common stock upon public offering (2017); initial public offering (2016) (in shares)	4,059,500
Stock-based compensation expense		4,241,000				4,241,000
Issuance of common stock upon the exercise of options and purchases under employee stock purchase plan		1,007,000				1,007,000
Issuance of common stock upon the exercise of options and purchases under employee stock purchase plan (in shares)	306,526
Other comprehensive gain (loss)			239,000			239,000
Net loss				(36,957,000)		(36,957,000)
Balance at Dec. 31, 2017		$ 222,188,000	$ (6,000)	$ (101,550,000)		$ 120,632,000
Balance (in shares) at Dec. 31, 2017	21,088,306					21,088,306