Summary of Significant Accounting Policies - Revenue Recognition (Details) $ in Thousands		1 Months Ended			3 Months Ended	6 Months Ended	12 Months Ended
	May 24, 2017 USD ($)	Dec. 31, 2017 USD ($)	Feb. 29, 2016 USD ($)	Mar. 31, 2015 item	Dec. 31, 2017 USD ($)	Jun. 30, 2018 USD ($) item	Dec. 31, 2010 USD ($) 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
Kadcyla | Minimum
Summary of Significant Accounting Policies
Period to earn royalty payments						10 years
Kadcyla | Maximum
Summary of Significant Accounting Policies
Period to earn royalty payments						12 years
Amgen
Summary of Significant Accounting Policies
Number of single-target licenses						2
Oxford BioTherapeutics Ltd Member | Amgen
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						1
Potential milestone payment | $			$ 160,000
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						5	6
Potential milestone payment | $							$ 199,500
Roche
Summary of Significant Accounting Policies
Number of single-target licenses						5
Sanofi
Summary of Significant Accounting Policies
Number of single-target licenses						5
Potential milestone payment | $						$ 0
Takeda
Summary of Significant Accounting Policies
Number of single-target licenses				2		1
Debiopharm
Summary of Significant Accounting Policies
Number of single-target licenses						1
Potential milestone payment | $	$ 5,000	$ 4,500			$ 5,000	$ 0