Summary of Significant Accounting Policies (Details)	1 Months Ended								3 Months Ended
	Oct. 31, 2015 USD ($)	Dec. 31, 2013 USD ($)	May. 31, 2013 USD ($) item	Dec. 31, 2012 USD ($) item	Dec. 31, 2011 item	Nov. 30, 2011 item	Nov. 30, 2009 USD ($) item	Sep. 30, 2009 USD ($) item	Sep. 30, 2015 USD ($) item	Sep. 30, 2014 USD ($) item	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)
Summary of Significant Accounting Policies
Number of types of licensing and development agreements with collaborative partners									2
License and milestone fees | $									$ 6,070,000	$ 6,234,000
Financial Instruments and Concentration of Credit Risk
Number of financial institutions in the U.S. in which cash and cash equivalents are primarily maintained									3
Number of marketable securities held by entity | $									$ 0		$ 0
Cash and cash equivalents | $									$ 247,843,000	121,798,000	$ 278,109,000	$ 142,261,000
Development and Commercialization License
Summary of Significant Accounting Policies
Average involvement period over which the upfront payments on single-target licenses are amortized									6 years 6 months
Difference between the full cost to manufacture and amounts received from collaborators for preclinical and clinical materials | $									$ 7,000,000	3,100,000
Number of types of milestone payments under collaborative arrangements									3
Number of quarters in arrear for revenue recognition									1
Development and Commercialization License | Minimum
Summary of Significant Accounting Policies
Period to earn royalty payments									10 years
Development and Commercialization License | Maximum
Summary of Significant Accounting Policies
Period to earn royalty payments									12 years
Development and Commercialization License | Kadcyla | Minimum
Summary of Significant Accounting Policies
Period to earn royalty payments									10 years
Development and Commercialization License | Kadcyla | Maximum
Summary of Significant Accounting Policies
Period to earn royalty payments									12 years
Right-to-test agreement | Minimum
Summary of Significant Accounting Policies
Average period over which upfront payments are deferred and recognized									12 months
Right-to-test agreement | Maximum
Summary of Significant Accounting Policies
Average period over which upfront payments are deferred and recognized									18 months
Amgen | Development and Commercialization License | Exclusive license
Summary of Significant Accounting Policies
Number of single-target licenses						3			4
License exercise fee | $				$ 1,000,000			$ 1,000,000	$ 1,000,000
Amgen | Development and Commercialization License | Non-exclusive license
Summary of Significant Accounting Policies
Number of single-target licenses			1
License exercise fee | $			$ 500,000
Amgen | Right-to-test agreement | Exclusive license
Summary of Significant Accounting Policies
Number of single-target licenses				3			3	3
Bayer HealthCare | Development and Commercialization License | Exclusive license
Summary of Significant Accounting Policies
Number of single-target licenses									1
Biotest | Development and Commercialization License | Exclusive license
Summary of Significant Accounting Policies
Number of single-target licenses									1
Lilly | Development and Commercialization License | Phase I clinical trial
Summary of Significant Accounting Policies
Number of single-target licenses									2
Lilly | Development and Commercialization License | Exclusive license
Summary of Significant Accounting Policies
Number of single-target licenses									3
Lilly | Right-to-test agreement
Summary of Significant Accounting Policies
Number of single-target licenses					3
Novartis | Development and Commercialization License | Exclusive license
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 | Development and Commercialization License | Exclusive license
Summary of Significant Accounting Policies
Number of single-target licenses									5
Sanofi | Development and Commercialization License
Summary of Significant Accounting Policies
Number of related targets									1
License exercise fee | $		$ 2,000,000
License and milestone fees | $										$ 1,700,000
Sanofi | Development and Commercialization License | Exclusive license
Summary of Significant Accounting Policies
Number of single-target licenses									1
Number of licenses to multiple individual targets									1
Oxford BioTherapeutics Ltd Member | Amgen | Development and Commercialization License | Exclusive license
Summary of Significant Accounting Policies
Number of single-target licenses										1
Subsequent event | Sanofi | Development and Commercialization License | Phase I clinical trial
Summary of Significant Accounting Policies
License and milestone fees | $	$ 2,000,000