Summary of Significant Accounting Policies - Revenue Recognition (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2020 USD ($) item	Dec. 31, 2010 item
Bayer
Summary of Significant Accounting Policies
Number of single-target licenses	1
CytomX
Summary of Significant Accounting Policies
Number of single-target licenses	2
Debiopharm
Summary of Significant Accounting Policies
Number of single-compound licenses	1
Future milestones or royalties to be received | $	$ 0
Fusion Pharmaceuticals
Summary of Significant Accounting Policies
Number of single-compound licenses	1
Novartis
Summary of Significant Accounting Policies
Number of single-target licenses	5	6
Oxford BioTherapeutics Ltd Member
Summary of Significant Accounting Policies
Number of single-target licenses	1
Roche
Summary of Significant Accounting Policies
Number of single-target licenses	5
Sanofi
Summary of Significant Accounting Policies
Number of single-target licenses	5
Future milestones or royalties to be received | $	$ 0