| Document and Entity Information | Dec. 04, 2020 |
| Document and Entity Information [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Dec. 4, 2020 |
| 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, State or Province | CA |
| Entity Address, City or Town | Monrovia |
| Entity Address, Postal Zip Code | 91016 |
| City Area Code | 626 |
| Local Phone Number | 305-5900 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value $0.01 per share |
| Trading Symbol | XNCR |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001326732 |
| Amendment Flag | false |
