License Agreements (Details) - Genentech - USD ($) $ in Millions		12 Months Ended			43 Months Ended
	Jun. 17, 2016	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019
Clinical
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments upon achievement of specified clinical and regulatory milestones	$ 37.5
Regulatory
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments upon achievement of specified clinical and regulatory milestones	102.5
Commercial
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments upon achievement of specified clinical and regulatory milestones	175.0
Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments upon achievement of specified clinical and regulatory milestones	$ 315.0
License Agreement | Research and Development
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront fee paid		$ 0.0	$ 0.0		$ 12.5
First clinical milestone				$ 2.5