License and Collaboration Agreements - Additional Information (Details) - USD ($)		1 Months Ended							3 Months Ended		6 Months Ended	12 Months Ended
	Jan. 01, 2019	Jun. 30, 2019	Nov. 30, 2018	Aug. 31, 2018	Jul. 31, 2018	Dec. 31, 2017	Nov. 30, 2017	Jun. 30, 2017	Mar. 31, 2020	Mar. 31, 2019	Jun. 30, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	May 01, 2019	Oct. 31, 2018
University Of California San Francisco
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Licensing fees									$ 40,000
Annual licenses maintenance fee									40,000
University Of California San Francisco | IPO
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments																$ 140,000
Sanofi Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Payment received		$ 30,000,000	$ 10,000,000	$ 5,000,000	$ 10,000,000	$ 40,000,000
Royalty expiration period							10 years
Upfront payment recognized												$ 40,000,000
Aggregate milestone payments												25,000,000
Transaction price determined												65,000,000
Revenue recognized									0	$ 0
Deferred revenue									0					$ 0
Sanofi Agreement | Clinical Development Milestones
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments															$ 30,000,000
Revenue recognized											$ 30,000,000
Increase in transaction price											$ 30,000,000
Sanofi Agreement | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License agreement milestone payment receive regulatory milestones and commercial milestones							$ 765,000,000
AbbVie Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Payment received								$ 15,000,000					$ 15,000,000
Revenue recognized									$ 0	$ 5,200,000
Research and development period								2 years
AbbVie Agreement | ASC 606
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized												9,800,000
AbbVie Agreement | ASC 606 | ASC 605
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized												$ 9,400,000
AbbVie Agreement | ASC 606 | Adjustments
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Decrease in accumulated deficit	$ 400,000
Decrease in deferred revenue	$ 400,000
AbbVie Agreement | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Extension of research and development period								6 months