Formation and Business of the Company - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended			6 Months Ended	12 Months Ended
	Oct. 03, 2016	Nov. 03, 2015	Jan. 31, 2017	Nov. 30, 2016	Aug. 31, 2014	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Accumulated deficit						$ (102,229)	$ (77,837)
Cash and cash equivalents						117,305	135,797	$ 87,615	$ 112,265
Short-term investments						16,011	4,072
Long-term investments						31,986	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		6,253,125
Shares issued, price per share		$ 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	4,370,000
Shares issued, price per share	$ 15.00
Net proceeds from issuance of common stock	$ 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