Condensed Consolidated Statements of Changes in Redeemable Convertible Preferred Stock and Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit
Increase (Decrease) in Stockholders' Equity
Cumulative stock-based compensation adjustment	$ 0		$ 522	$ (522)
Redeemable convertible preferred stock beginning balance (in shares) at Dec. 31, 2016	55,422,002
Redeemable convertible preferred stock beginning balance at Dec. 31, 2016	$ 655,416
Increase (Decrease) in Temporary Equity [Roll Forward]
Accretion of redeemable convertible preferred stock	$ 175,915
Conversion of redeemable convertible preferred stock to common stock (in shares)	(55,422,002)
Conversion of redeemable convertible preferred stock to common stock	$ (831,331)
Redeemable convertible preferred stock ending balance (in shares) at Dec. 31, 2017	0
Redeemable convertible preferred stock ending balance at Dec. 31, 2017	$ 0
Common stock, outstanding, beginning balance (in shares) at Dec. 31, 2016		14,687,024
Total stockholders' deficit, beginning balance at Dec. 31, 2016	(563,734)	$ 15	0	(563,749)
Increase (Decrease) in Stockholders' Equity
Proceeds from initial public offering, net of underwriters' discounts/Issuance of common stock related to follow-on offering, net (in shares)		10,615,650
Proceeds from initial public offering, net of underwriters' discounts/Issuance of common stock related to follow-on offering, net	148,088	$ 10	148,078
Initial public offering costs	(3,708)		(3,708)
Exercise of stock options (in shares)		744,215
Exercise of stock options	3,001	$ 1	3,000
Stock-based compensation	11,369		11,369
Accretion of redeemable convertible preferred stock	(175,915)		(8,690)	(167,225)
Conversion of redeemable convertible preferred stock to common stock (in shares)		55,422,002
Conversion of redeemable convertible preferred stock to common stock	831,331	$ 55	213,781	617,495
Net loss	$ (15,002)			(15,002)
Common stock, outstanding, ending balance (in shares) at Dec. 31, 2017	81,468,891	81,468,891
Total stockholders' deficit, ending balance at Dec. 31, 2017	$ 235,430	$ 81	364,352	(129,003)
Redeemable convertible preferred stock ending balance (in shares) at Sep. 30, 2018	0
Redeemable convertible preferred stock ending balance at Sep. 30, 2018	$ 0
Increase (Decrease) in Stockholders' Equity
Proceeds from initial public offering, net of underwriters' discounts/Issuance of common stock related to follow-on offering, net (in shares)		4,836,336
Proceeds from initial public offering, net of underwriters' discounts/Issuance of common stock related to follow-on offering, net	107,593	$ 5	107,588
Issuance of common stock pursuant to employee stock purchase program (in shares)		187,076
Issuance of common stock pursuant to employee stock purchase program	$ 3,672		3,672
Exercise of stock options (in shares)	2,653,008	2,653,008
Exercise of stock options	$ 13,728	$ 3	13,725
Issuance of common stock pursuant to restricted stock units (in shares)		127,746
Issuance of common stock pursuant to restricted stock units	0	$ 0
Restricted stock surrendered for employees' tax liability (in shares)		(38,238)
Restricted stock surrendered for employees' tax liability	(705)		(705)
Equity component of convertible senior notes, net	27,951		27,951
Stock-based compensation	14,835		14,835
Net loss	$ (29,756)			(29,756)
Common stock, outstanding, ending balance (in shares) at Sep. 30, 2018	89,234,819	89,234,819
Total stockholders' deficit, ending balance at Sep. 30, 2018	$ 372,748	$ 89	$ 531,418	$ (158,759)