Formation and Business of the Company - Additional Information (Details) $ / shares in Units, $ in Thousands				1 Months Ended			9 Months Ended	12 Months Ended
	Aug. 14, 2017 USD ($) $ / shares shares	Oct. 03, 2016 USD ($) $ / shares shares	Nov. 03, 2015 USD ($) $ / shares shares	Jan. 31, 2017 USD ($)	Nov. 30, 2016 USD ($)	Aug. 31, 2014 USD ($)	Sep. 30, 2017 USD ($) Offering	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of follow-on public offerings | Offering							2
Net proceeds from issuance of common stock							$ 133,925
Accumulated deficit							(116,402)	$ (77,837)
Cash and cash equivalents							227,184	135,797	$ 77,120	$ 112,265
Short-term investments							19,993	4,072
Long-term investments							36,003	12,002
Series A, A-1 and Series B Redeemable Convertible Preferred Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Net proceeds from sale of shares of convertible stock							$ 93,900
IPO
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Issuance of common stock | shares			6,253,125
Shares issued, price per share | $ / shares			$ 10.00
Net proceeds from initial public offering			$ 55,600
Follow-on Offering
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Issuance of common stock | shares	4,025,000	4,370,000
Shares issued, price per share | $ / shares	$ 35.50	$ 15.00
Net proceeds from issuance of common stock	$ 133,800	$ 61,100
Sanofi (Aventis Inc.) | Collaborative Agreement
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Collaboration agreement, amount received					$ 105,000
Upfront cash received under collaboration agreement					35,000	$ 35,000
Revenue recognized under milestone-based payments					$ 25,000			$ 25,000
Continuation payment, amount received				$ 45,000