v3.19.3
Document and Entity Information - shares
9 Months Ended
Sep. 30, 2019
Oct. 31, 2019
Document and Entity Information    
Document Type 10-Q  
Document Quarterly Report true  
Document Transition Report false  
Document Period End Date Sep. 30, 2019  
Entity File Number 001-36182  
Entity Registrant Name Xencor Inc  
Entity Incorporation, State or Country Code DE  
Entity Tax Identification Number 20-1622502  
Entity Address, Address Line One 111 West Lemon Avenue  
Entity Address, City or Town Monrovia  
Entity Address, State or Province CA  
Entity Address, Postal Zip Code 91016  
City Area Code 626  
Local Phone Number 305-5900  
Title of 12(b) Security Common Stock, par value $0.01 per share  
Trading Symbol XNCR  
Security Exchange Name NASDAQ  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Large Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   56,721,172
Entity Central Index Key 0001326732  
Amendment Flag false  
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q3