License Agreements - Narrative (Details) - Genentech Inc - USD ($)	1 Months Ended	3 Months Ended		6 Months Ended		49 Months Ended
	Jun. 30, 2016	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020
License Agreement | Research and Development
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront fee paid		$ 0	$ 0	$ 0	$ 0	$ 12,500,000
Clinical
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments upon achievement of specified clinical and regulatory milestones	$ 37,500,000
Regulatory
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments upon achievement of specified clinical and regulatory milestones	102,500,000
Commercial
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments upon achievement of specified clinical and regulatory milestones	175,000,000.0
Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments upon achievement of specified clinical and regulatory milestones	$ 315,000,000.0