| Document and Entity Information | Apr. 28, 2021 |
| Document and Entity Information [Abstract] | |
| Document Type | 8-K/A |
| Document Period End Date | Apr. 28, 2021 |
| Entity File Number | 001-39036 |
| Entity Registrant Name | Alerus Financial Corporation |
| Entity Incorporation, State or Country Code | DE |
| Entity Tax Identification Number | 45-0375407 |
| Entity Address, Address Line One | 401 Demers Avenue |
| Entity Address, City or Town | Grand Forks |
| Entity Address, State or Province | ND |
| Entity Address, Postal Zip Code | 58201 |
| City Area Code | 701 |
| Local Phone Number | 795-3200 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $1.00 par value per share |
| Trading Symbol | ALRS |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
| Entity Central Index Key | 0000903419 |
| Amendment Flag | false |
