Summary of Significant Accounting Policies - Revenue Recognition (Details)	1 Months Ended						12 Months Ended
	Oct. 31, 2018 item	Dec. 31, 2015 item	Mar. 31, 2015 USD ($) item	Oct. 31, 2013	May 31, 2013 item	Mar. 31, 2013 item	Dec. 31, 2018 USD ($) item	Dec. 31, 2011 item	Dec. 31, 2010 item	Dec. 31, 2000 USD ($) item	Jan. 31, 2014 USD ($)
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
Amgen/Oxford BioTherapeutics
Summary of Significant Accounting Policies
Number of single-target licenses		4			1					3
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							1
Potential milestone payment | $											$ 160,000,000
Fusion Pharmaceuticals
Summary of Significant Accounting Policies
Number of single-target licenses							1
Potential milestone payment | $							$ 500,000
Lilly
Summary of Significant Accounting Policies
Number of single-target licenses	3							3
Novartis
Summary of Significant Accounting Policies
Number of single-target licenses						1	5		6
Term of agreement				1 year					3 years
Roche
Summary of Significant Accounting Policies
Number of single-target licenses							5
Roche | Undisclosed Target
Summary of Significant Accounting Policies
Potential milestone payment | $										$ 38,000,000
Sanofi
Summary of Significant Accounting Policies
Number of single-target licenses							5
Takeda
Summary of Significant Accounting Policies
Number of single-target licenses							1
Term of agreement			3 years
Potential milestone payment | $			$ 210,000,000
Takeda | Maximum
Summary of Significant Accounting Policies
Number of single-target licenses			2