Summary of Significant Accounting Policies - Revenue Recognition (Details)	3 Months Ended
	Mar. 31, 2020 item
Minimum
Summary of Significant Accounting Policies
Period to earn royalty payments	10 years
Maximum
Summary of Significant Accounting Policies
Period to earn royalty payments	12 years
Oxford BioTherapeutics Ltd Member
Summary of Significant Accounting Policies
Number of single-target licenses	1
Bayer
Summary of Significant Accounting Policies
Number of single-target licenses	1
Biotest
Summary of Significant Accounting Policies
Number of single-target licenses	1
CytomX
Summary of Significant Accounting Policies
Number of single-target licenses	2
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
Roche
Summary of Significant Accounting Policies
Number of single-target licenses	5
Sanofi
Summary of Significant Accounting Policies
Number of single-target licenses	5
Takeda
Summary of Significant Accounting Policies
Number of single-target licenses	1
Debiopharm
Summary of Significant Accounting Policies
Number of single-compound licenses	1