v3.3.1.900
Document and Entity Information - USD ($)
12 Months Ended
Dec. 31, 2015
Feb. 26, 2016
Jun. 30, 2015
Document and Entity Information      
Entity Registrant Name Xencor Inc    
Entity Central Index Key 0001326732    
Document Type 10-K    
Document Period End Date Dec. 31, 2015    
Amendment Flag false    
Current Fiscal Year End Date --12-31    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Filer Category Accelerated Filer    
Entity Public Float     $ 490,431,477
Entity Common Stock, Shares Outstanding   40,582,039  
Document Fiscal Year Focus 2015    
Document Fiscal Period Focus FY