Collaboration and License Agreement - Additional Information (Details) - Sanofi (Aventis Inc.) - Collaborative Agreement	1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	Apr. 30, 2015 USD ($)	Aug. 31, 2014 USD ($) Program $ / shares shares	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash received under collaboration agreement		$ 35,000,000
Initial equity investment		10,000,000
Eligible to receive milestone payment		$ 25,000,000
Period for research and clinical activities		4 years
Number of research programs | Program		3
Deferred revenue		$ 35,000,000	$ 3,500,000		$ 3,500,000		$ 14,200,000
Revenue recognized under collaboration agreement			3,600,000	$ 3,600,000	$ 10,600,000	$ 10,600,000
One-time non-refundable non-creditable cash payment		45,000,000
Project continuation, additional payment		40,000,000
Project continuation, additional payment description					an additional $40.0 million in connection with the purchase of the Company’s preferred stock, assuming (i) the Company had not previously closed either a Qualified IPO (at which time this obligation would terminate) or a private financing prior to a Qualified IPO and (ii) Sanofi had not previously purchased shares of the Company’s stock pursuant to such rights to purchase the Company’s capital stock in accordance with the terms of the Collaboration Agreement.
Time restricted rights to purchase stock, value					$ 40,000,000
Convertible preferred stock at discount			$ 40,000,000		$ 40,000,000
Redeemable convertible preferred stock call option liability fair value		$ 700,000
Change in fair value of redeemable convertible preferred stock call option liability							$ (300,000)	$ (400,000)
Series A-1 Redeemable Convertible Preferred Stock
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Issuance of common stock | shares		6,666,667
Fair value of stock per share price | $ / shares		$ 1.50
Series B Redeemable Convertible Preferred Stock
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Project continuation, additional payment	$ 35,000,000
Decrease in project continuation additional payment	$ 5,000,000
Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from collaboration agreement		$ 200,000,000
Project continuation payment		85,000,000
Funding from approved in-kind research and clinical activities		$ 45,000,000