Revenue - Narrative (Details) - USD ($)	Feb. 08, 2018	Sep. 30, 2018	Dec. 31, 2017
Deferred Revenue Arrangement [Line Items]
Current deferred revenues		$ 31,920,000	$ 37,153,000
Non-current deferred revenue		60,770,000	$ 46,542,000
Seattle Genetics | License and Collaboration Agreement
Deferred Revenue Arrangement [Line Items]
Upfront fees received	$ 30,000,000
Estimated reimbursements for research, development and manufacturing services	4,900,000
Maximum tiered royalties on net sales	$ 1,200,000,000
Period after effective date agreements may be terminated	12 months
Advance notice period for contract termination	90 days
Allocable arrangement consideration	$ 34,900,000
Current deferred revenues		7,800,000
Non-current deferred revenue		$ 16,300,000
Seattle Genetics | License and Collaboration Agreement | Program Pivotal Study Initiated
Deferred Revenue Arrangement [Line Items]
Advance notice period for contract termination	180 days
Seattle Genetics | License and Collaboration Agreement | Material Breach of Contract
Deferred Revenue Arrangement [Line Items]
Advance notice period for contract termination	90 days
Seattle Genetics | License and Collaboration Agreement | Material Breach of Contract due to Diligence Obligations
Deferred Revenue Arrangement [Line Items]
Advance notice period for contract termination	90 days