| Document And Entity Information | Jan. 28, 2021 |
| Cover [Abstract] | |
| Entity Registrant Name | Allegiance Bancshares, Inc. |
| Document Type | 8-K |
| Amendment Flag | false |
| Entity Central Index Key | 0001642081 |
| Document Period End Date | Jan. 28, 2021 |
| Entity Emerging Growth Company | false |
| Entity File Number | 001-37585 |
| Entity Incorporation, State or Country Code | TX |
| Entity Tax Identification Number | 26-3564100 |
| Entity Address, Address Line One | 8847 West Sam Houston Parkway N. |
| Entity Address, Address Line Two | Suite 200 |
| Entity Address, City or Town | Houston |
| Entity Address, State or Province | TX |
| Entity Address, Postal Zip Code | 77040 |
| City Area Code | 281 |
| Local Phone Number | 894-3200 |
| 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 $1.00 per share |
| Trading Symbol | ABTX |
| Security Exchange Name | NASDAQ |
