| Document and Entity Information | Jun. 30, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Jun. 30, 2021 |
| Entity Registrant Name | KBR, Inc. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-33146 |
| Entity Tax Identification Number | 20-4536774 |
| Entity Address, Address Line One | 601 Jefferson Street |
| Entity Address, Address Line Two | Suite 3400 |
| Entity Address, City or Town | Houston |
| Entity Address, State or Province | TX |
| Entity Address, Postal Zip Code | 77002 |
| City Area Code | 713 |
| Local Phone Number | 753-2000 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001357615 |
| Title of 12(b) Security | Common Stock, $0.001 par value |
| Trading Symbol | KBR |
| Security Exchange Name | NYSE |
