Consolidated Statements of Convertible Preferred Stock and Stockholders’ Equity (Deficit) - USD ($) $ in Thousands	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Total
Temporary equity beginning balance at Dec. 31, 2015				$ 194,869
Temporary equity beginning balance (in shares) at Dec. 31, 2015				14,500,938
Temporary equity ending balance at Dec. 31, 2016				$ 194,869
Temporary equity ending balance (in shares) at Dec. 31, 2016				14,500,938
Beginning balance at Dec. 31, 2015	$ 7	$ 1,727	$ (83,668)	$ (81,934)
Beginning balance (in shares) at Dec. 31, 2015	66,565,002
Convertible Preferred Stock
Issuance of common stock upon exercise of stock options		402		402
Issuance of common stock upon exercise of stock options (in shares)	530,126
Share-based compensation		3,018		3,018
Net income (loss)			(54,886)	(54,886)
Ending balance at Dec. 31, 2016	$ 7	5,147	(138,554)	(133,400)
Ending balance (in shares) at Dec. 31, 2016	67,095,128
Convertible Preferred Stock
Redemption of common stock upon conversion				$ (194,869)
Redemption of common stock upon conversion (in shares)				(14,500,938)
Temporary equity ending balance (in shares) at Sep. 30, 2017				0
Convertible Preferred Stock
Issuance of common stock upon exercise of stock options		816		$ 816
Issuance of common stock upon exercise of stock options (in shares)	788,498
Issuance of common stock upon acquisition		373		373
Issuance of common stock upon acquisition (in shares)	42,687
Issuance of convertible notes		19,567		19,567
Issuance of common stock upon initial public offering, net of offering costs	$ 3	278,007		278,010
Issuance of common stock (in shares)	30,000,000
Issuance of common stock upon conversion of convertible notes	$ 1	62,084		62,085
Issuance of common stock upon conversion of convertible notes (in shares)	7,023,201
Issuance of common stock upon conversion of Series A, B, C, and D convertible preferred stock	$ 8	194,861		194,869
Issuance of common stock upon conversion of Series A, B, C, and D convertible preferred stock (in shares)	85,190,551
Share-based compensation		8,900		8,900
Net income (loss)			(171,023)	(171,023)
Ending balance at Sep. 30, 2017	$ 19	$ 569,755	$ (309,577)	$ 260,197
Ending balance (in shares) at Sep. 30, 2017	190,140,065