| Document and Entity Information - USD ($) | 12 Months Ended | ||
| Dec. 31, 2020 | Mar. 31, 2021 | Jun. 30, 2020 | |
| Cover [Abstract] | |||
| Entity Registrant Name | ALTABANCORP | ||
| Entity Central Index Key | 0001636286 | ||
| Document Type | 10-K/A | ||
| Document Period End Date | Dec. 31, 2020 | ||
| Amendment Flag | true | ||
| Document Fiscal Year Focus | 2020 | ||
| Trading Symbol | ALTA | ||
| Document Fiscal Period Focus | FY | ||
| Current Fiscal Year End Date | --12-31 | ||
| Entity Well-known Seasoned Issuer | No | ||
| Entity Current Reporting Status | Yes | ||
| Entity Voluntary Filers | No | ||
| Entity Filer Category | Accelerated Filer | ||
| Entity Small Business | false | ||
| Entity Emerging Growth Company | false | ||
| Entity Shell Company | false | ||
| Entity Common Stock, Shares Outstanding | 18,873,921 | ||
| Entity Public Float | $ 270,352,838 | ||
| Entity File Number | 001-37416 | ||
| 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 | ||
| Entity Interactive Data Current | Yes | ||
| Title of 12(b) Security | Common Stock, par value $0.01 per share | ||
| Security Exchange Name | NASDAQ | ||
| Entity Incorporation, State or Country Code | UT | ||
| Document Transition Report | false | ||
| Document Annual Report | true | ||
| Amendment Description | AltabancorpTM (“ALTA” or the “Company”) is filing this Amendment No. 1 on Form 10-K/A (this “Amendment”) to amend our Annual Report on Form 10-K for the fiscal year ended December 31, 2020 that was filed with the Securities and Exchange Commission (the “SEC”) on March 15, 2021 (the “Original Filing”). We are filing this Amendment for the purpose of including information required by Part III of Form 10-K as discussed further below. This information was previously omitted from the Original Filing in reliance on General Instruction G(3) to Form 10-K, which permits the information in Part III to be incorporated in Form 10-K by reference to our definitive proxy statement if such statement is filed no later than 120 days after the end of our fiscal year. Our definitive proxy statement for our 2021 Annual Meeting of Shareholders will not be filed within 120 days of December 31, 2020. The inclusion of the definitive proxy statement on the cover page of the Original Filing as a document incorporated by reference has been deleted. This Amendment amends and restates in their entirety Items 10, 11, 12, 13 and 14 of Part III of the Original Filing. | ||
| ICFR Auditor Attestation Flag | true |
