| Document and Entity Information | Jan. 27, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Jan. 27, 2021 |
| Entity Registrant Name | Altabancorp |
| Entity Central Index Key | 0001636286 |
| Entity Emerging Growth Company | false |
| Entity File Number | 001-37416 |
| Entity Incorporation, State or Country Code | UT |
| Entity Tax Identification Number | 87-0622021 |
| Entity Address, Address Line One | 1 East Main Street |
| Entity Address, City or Town | American Fork |
| Entity Address, State or Province | UT |
| Entity Address, Postal Zip Code | 84003 |
| City Area Code | 801 |
| Local Phone Number | 642-3998 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of each class | Common Stock, par value $0.01 per share |
| Trading Symbol(s) | ALTA |
| Name of each exchange on which registered | NASDAQ |
