Summary of Significant Accounting Policies - Revenue Recognition (Details) - item	1 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2013	Dec. 31, 2019	Dec. 31, 2011	Dec. 31, 2010
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-target licenses			1
Lilly
Summary of Significant Accounting Policies
Number of single-target licenses				3
Novartis
Summary of Significant Accounting Policies
Number of single-target licenses		1	5		6
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
Takeda | Maximum
Summary of Significant Accounting Policies
Number of single-target licenses	2
Debiopharm
Summary of Significant Accounting Policies
Number of single-compound licenses			1