Collaboration Agreements and Supply Agreements - Summary of Recognized Revenue (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Revenue Recognition Multiple Deliverable Arrangements [Line Items]        
Total revenue $ 7,836 $ 17,499 $ 19,333 $ 47,701
Collaboration Revenue        
Revenue Recognition Multiple Deliverable Arrangements [Line Items]        
Total revenue 6,924 17,499 13,955 47,701
Collaboration Agreements and Supply Agreements        
Revenue Recognition Multiple Deliverable Arrangements [Line Items]        
Total revenue 7,836 17,499 19,333 47,701
Total other revenue—related parties 912 0 5,378 0
Collaboration Agreements and Supply Agreements | Collaboration Revenue, Amortization of Up-front Payment | Celgene        
Revenue Recognition Multiple Deliverable Arrangements [Line Items]        
Total revenue 1,655 2,642 4,912 16,355
Collaboration Agreements and Supply Agreements | Collaboration Revenue, Amortization of Up-front Payment | Merck        
Revenue Recognition Multiple Deliverable Arrangements [Line Items]        
Total revenue 2,818 0 2,818 0
Collaboration Agreements and Supply Agreements | Collaboration Revenue, Amortization of Up-front Payment | Merck KGaA, Darmstadt, Germany “EMD Serono”)        
Revenue Recognition Multiple Deliverable Arrangements [Line Items]        
Total revenue 1,038 1,030 3,104 3,090
Collaboration Agreements and Supply Agreements | Collaboration Revenue, Research and Development Services | Celgene        
Revenue Recognition Multiple Deliverable Arrangements [Line Items]        
Total revenue 5 0 103 0
Collaboration Agreements and Supply Agreements | Collaboration Revenue, Research and Development Services | Merck        
Revenue Recognition Multiple Deliverable Arrangements [Line Items]        
Total revenue 696 0 696 0
Collaboration Agreements and Supply Agreements | Collaboration Revenue, Research and Development Services | Merck KGaA, Darmstadt, Germany “EMD Serono”)        
Revenue Recognition Multiple Deliverable Arrangements [Line Items]        
Total revenue 712 715 2,322 2,319
Collaboration Agreements and Supply Agreements | Collaboration Revenue, Milestones and Contingent Payments | Celgene        
Revenue Recognition Multiple Deliverable Arrangements [Line Items]        
Total revenue 0 13,112 0 25,937
Collaboration Agreements and Supply Agreements | Collaboration Revenue        
Revenue Recognition Multiple Deliverable Arrangements [Line Items]        
Total revenue 6,924 17,499 13,955 47,701
Collaboration Agreements and Supply Agreements | Collaboration Revenue | Celgene        
Revenue Recognition Multiple Deliverable Arrangements [Line Items]        
Total revenue 1,660 15,754 5,015 42,292
Collaboration Agreements and Supply Agreements | Collaboration Revenue | Merck        
Revenue Recognition Multiple Deliverable Arrangements [Line Items]        
Total revenue 3,514 0 3,514 0
Collaboration Agreements and Supply Agreements | Collaboration Revenue | Merck KGaA, Darmstadt, Germany “EMD Serono”)        
Revenue Recognition Multiple Deliverable Arrangements [Line Items]        
Total revenue 1,750 1,745 5,426 5,409
Collaboration Agreements and Supply Agreements | Development and Manufacturing Services and Clinical Product Supply | Celgene        
Revenue Recognition Multiple Deliverable Arrangements [Line Items]        
Total other revenue—related parties 330 0 3,894 0
Collaboration Agreements and Supply Agreements | Supply and Other | SutroVax        
Revenue Recognition Multiple Deliverable Arrangements [Line Items]        
Total other revenue—related parties $ 582 $ 0 $ 1,484 $ 0