Summary of Significant Accounting Policies - Revenue Recognition (Details) $ in Thousands	1 Months Ended				9 Months Ended	12 Months Ended
	May 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Feb. 29, 2016 USD ($)	Mar. 31, 2015 item	Sep. 30, 2018 USD ($) 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					1
Potential milestone payment | $					$ 34,000
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
Potential milestone payment | $		$ 50,000
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				$ 0