Summary of Significant Accounting Policies - Revenue (Details) shares in Millions		1 Months Ended	9 Months Ended
	Oct. 11, 2017 USD ($) shares	Nov. 30, 2017 USD ($) employee	Sep. 30, 2017 USD ($) item	Sep. 30, 2016 USD ($)	May 24, 2017 USD ($)	Feb. 29, 2016 USD ($)	Dec. 31, 2008 USD ($)
Summary of Significant Accounting Policies
Issuance of common stock in a public offering, net of issuance costs (in shares) | shares	16.7
Proceeds from common stock issuance, net | $	$ 101,600,000
Number of employees with share-based equity modifications | employee		2
Difference between full cost and amounts received | $			$ 1,100,000	$ 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
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			1
Oxford BioTherapeutics Ltd Member | Amgen
Summary of Significant Accounting Policies
Number of single-target licenses			1
Debiopharm
Summary of Significant Accounting Policies
Milestone revenue recognized | $		$ 5,000,000
Number of single-target licenses			1
Potential milestone payments | $					$ 5,000,000
Royalty revenue | $			$ 0