Summary of Significant Accounting Policies - Revenue Recognition (Details)		1 Months Ended	3 Months Ended	12 Months Ended
	May 01, 2018 USD ($)	Mar. 31, 2015 item	Mar. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($) item	Jan. 31, 2018 USD ($)	May 24, 2017 USD ($)	Feb. 29, 2016 USD ($)
Summary of Significant Accounting Policies
Portion of upfront payment recognized | $			$ 11,540,000
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 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	6
Number of licenses to two related targets			1
Number of related targets			2
Number of related targets licenses on exclusive basis			1
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 payments | $			$ 0
Takeda
Summary of Significant Accounting Policies
Number of single-target licenses		2	1
Portion of upfront payment recognized | $			$ 10,900,000
Takeda | Subsequent event
Summary of Significant Accounting Policies
Portion of upfront payment recognized | $	$ 5,000,000
Debiopharm
Summary of Significant Accounting Policies
Number of single-target licenses			1
Potential milestone payments | $			$ 0	$ 5,000,000	$ 4,500,000	$ 5,000,000
Portion of upfront payment recognized | $			$ 500,000