Consolidated Statements of Changes in Redeemable Convertible Preferred Stock and Stockholders' Equity/(Deficit) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit
Redeemable convertible preferred stock beginning balance (in shares) at Dec. 31, 2014	55,422,002
Redeemable convertible preferred stock beginning balance at Dec. 31, 2014	$ 497,699
Increase (Decrease) in Temporary Equity [Roll Forward]
Redeemable convertible preferred stock issuance costs	(9)
Accretion of redeemable convertible preferred stock	$ 102,224
Redeemable convertible preferred stock ending balance (in shares) at Dec. 31, 2015	55,422,002
Redeemable convertible preferred stock ending balance at Dec. 31, 2015	$ 599,914
Common stock, outstanding, beginning balance (in shares) at Dec. 31, 2014		12,629,479
Total stockholders' deficit, beginning balance at Dec. 31, 2014	(370,595)	$ 13	$ 0	$ (370,608)
Increase (Decrease) in Stockholders' Equity
Exercise of stock options (in shares)		1,430,122
Exercise of stock options	1,731	$ 1	1,730
Stock-based compensation	5,611		5,611
In-substance dividend issued in tender offer	0		2,659	(2,659)
Accretion of redeemable convertible preferred stock	(102,224)		(10,000)	(92,224)
Net income (loss)	(30,236)			(30,236)
Common stock, outstanding, ending balance (in shares) at Dec. 31, 2015		14,059,601
Total stockholders' deficit, ending balance at Dec. 31, 2015	(495,713)	$ 14	0	(495,727)
Increase (Decrease) in Temporary Equity [Roll Forward]
Accretion of redeemable convertible preferred stock	$ 55,502
Redeemable convertible preferred stock ending balance (in shares) at Dec. 31, 2016	55,422,002
Redeemable convertible preferred stock ending balance at Dec. 31, 2016	$ 655,416
Increase (Decrease) in Stockholders' Equity
Exercise of stock options (in shares)		627,423
Exercise of stock options	1,495	$ 1	1,494
Stock-based compensation	8,512		8,512
Accretion of redeemable convertible preferred stock	(55,502)		(10,006)	(45,496)
Net income (loss)	$ (22,526)			(22,526)
Common stock, outstanding, ending balance (in shares) at Dec. 31, 2016	14,687,024	14,687,024
Total stockholders' deficit, ending balance at Dec. 31, 2016	$ (563,734)	$ 15	0	(563,749)
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
Increase (Decrease) in Stockholders' Equity
Proceeds from initial public offering, net of underwriters' discounts (in shares)		10,615,650
Proceeds from initial public offering, net of underwriters' discounts	148,088	$ 10	148,078
Initial public offering costs	$ (3,708)		(3,708)
Exercise of stock options (in shares)	744,215	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 income (loss)	$ (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)
Increase (Decrease) in Stockholders' Equity
Cumulative stock-based compensation adjustment	$ 0		$ 522	$ (522)