Summary of Significant Accounting Policies - Revenue (Details)	6 Months Ended
	Jun. 30, 2017 USD ($) item	Jun. 30, 2016 USD ($)	May 24, 2017 USD ($)	Feb. 29, 2016 USD ($)	Dec. 31, 2008 USD ($)	Dec. 31, 2000 USD ($)
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 | $	$ 929,000	$ 2,800,000
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
Bayer
Summary of Significant Accounting Policies
Number of single-target licenses	1
Potential milestone payments | $					$ 170,500,000
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 payments | $				$ 160,000,000
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
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
Roche | Kadcyla
Summary of Significant Accounting Policies
Potential milestone payments | $						$ 44,000,000
Sanofi
Summary of Significant Accounting Policies
Number of single-target licenses	5
Potential milestone payments | $	$ 0
Takeda
Summary of Significant Accounting Policies
Number of related targets	1
Oxford BioTherapeutics Ltd Member | Amgen
Summary of Significant Accounting Policies
Number of single-target licenses	1
Debiopharm
Summary of Significant Accounting Policies
Number of single-target licenses	1
Potential milestone payments | $			$ 5,000,000
Royalty revenue | $	$ 0