Summary of Significant Accounting Policies - Revenue (Details)	3 Months Ended
	Mar. 31, 2017 USD ($) item	Mar. 31, 2016 USD ($)
Summary of Significant Accounting Policies
Number of types of licensing and development agreements with collaborative partners	2
Difference between the full cost to manufacture and amounts received from collaborators for preclinical and clinical materials | $	$ 727,000	$ 3,000,000
Number of types of milestone payments under collaborative arrangements	3
Number of quarters in arrear for revenue recognition	1
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
Amgen | Oxford BioTherapeutics Ltd Member
Summary of Significant Accounting Policies
Number of related targets	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
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
Number of licenses to two related targets	1
Number of related targets	2
Roche
Summary of Significant Accounting Policies
Number of single-target licenses	5
Sanofi
Summary of Significant Accounting Policies
Number of single-target licenses	1
Number of licenses to multiple individual targets	1
Takeda
Summary of Significant Accounting Policies
Number of related targets	1