Collaboration and License Agreement - Additional Information (Details) - Sanofi (Aventis Inc.) - Collaborative Agreement	1 Months Ended			3 Months Ended		6 Months Ended		12 Months Ended
	Nov. 30, 2016 USD ($)	Apr. 30, 2015 USD ($)	Aug. 31, 2014 USD ($) Program $ / shares shares	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash received under collaboration agreement	$ 35,000,000		$ 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	$ 0		$ 0		$ 0
Revenue recognized under collaboration agreement				0	$ 3,500,000	$ 0	$ 7,100,000
Revenue recognized under milestone-based payments	$ 25,000,000							25,000,000
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 reduced by $5.0 million in connection with the purchase of the Company’s preferred stock, assuming the Company has not previously closed (i) either a Qualified IPO (at which time this obligation will terminate) or a private financing prior to a Qualified IPO and (ii) Sanofi has 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
Project continuation, revenue recognized				5,600,000		11,300,000
Project continuation, deferred revenue				33,800,000		33,800,000
Redeemable convertible preferred stock call option liability fair value			$ 700,000
Redeemable convertible preferred stock call option liability				$ 0		$ 0		$ 0
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
Common stock owned percentage				11.70%		11.70%
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