Significant Accounting Policies (Policies)	9 Months Ended
Significant Accounting Policies (Policies)	Sep. 30, 2020
Significant Accounting Policies
Organization	Organization Alerus Financial Corporation, or the Company, is a financial holding company organized under the laws of the state of Delaware. The Company and its subsidiaries operate as a diversified financial services company headquartered in Grand Forks, North Dakota. Through its subsidiary, Alerus Financial, National Association, or the Bank, the Company provides financial solutions to businesses and consumers through four distinct business lines—banking, retirement and benefit services, wealth management, and mortgage.
Initial Public Offering	Initial Public Offering On September 17, 2019, the Company sold 2,860,000 shares of common stock in its initial public offering. On September 25, 2019, the Company sold an additional 429,000 shares of common stock pursuant to the exercise in full, by the underwriters, of their option to purchase additional shares. The aggregate offering price for the shares sold by the Company was $69.1 million, and after deducting $4.7 million of underwriting discounts and $1.6 million of offering expenses paid to third parties, the Company received total net proceeds of $62.8 million.
Basis of Presentation	Basis of Presentation The accompanying unaudited consolidated financial statements and notes thereto of the Company have been prepared in accordance with the rules and regulations of the Securities and Exchange Commission, or SEC, and conform to practices within the banking industry and include all of the information and disclosures required by generally accepted accounting principles in the United States of America, or GAAP, for interim financial reporting. The accompanying unaudited consolidated financial statements reflect all adjustments (consisting only of normal recurring adjustments) that are necessary for a fair presentation of financial results for the interim periods presented. The results of operations for the interim periods are not necessarily indicative of the results for the full year or any other period. The Company has also evaluated all subsequent events for potential recognition and disclosure through the date of the filing of this Quarterly Report on Form 10-Q. These interim unaudited financial statements should be read in conjunction with the audited consolidated financial statements and the notes thereto as of and for the year ended December 31, 2019, included in the Company’s Annual Report on Form 10-K filed with the SEC on March 26, 2020.
Principles of Consolidation	Principles of Consolidation The accompanying unaudited consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries. The Company’s principal operating subsidiary is the Bank. In the normal course of business, the Company may enter into a transaction with a variable interest entity, or VIE. VIE’s are legal entities whose investors lack the ability to make decisions about the entity’s activities, or whose equity investors do not have the right to receive the residual returns of the entity. The applicable accounting guidance requires the Company to perform ongoing quantitative and qualitative analysis to determine whether it must consolidate any VIE. The Company does not have any ownership interest in, or exert any control, over any VIE, and thus no VIE’s are included in the consolidated financial statements.
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenue and expenses during the reporting period. Actual results could differ from those estimates. Material estimates that are particularly susceptible to significant change in the near term include the valuation of investment securities, determination of the allowance for loan losses, valuation of reporting units for the purpose of testing goodwill and other intangible assets for impairment, valuation of deferred tax assets, and fair values of financial instruments.
Reclassifications	Reclassifications Certain items previously reported have been reclassified to conform to the current period’s reporting format. Such reclassifications did not affect net income or stockholders’ equity.
Other Information	Other Information On March 11, 2020, the World Health Organization declared the spread of Coronavirus Disease 2019, or COVID-19, a worldwide pandemic. The COVID-19 pandemic is having significant effects on global markets, supply chains, businesses, and communities. Specific to the Company, COVID-19 has impacted various parts of its 2020 operations and financial results, including, but not limited to, additional loan loss reserves, costs for emergency preparedness, or potential shortages of personnel. Management believes the Company is taking appropriate actions to mitigate, to the extent possible, the negative impact. However, the full impact of COVID-19 is currently unknown and cannot be reasonably estimated as the events are continuing to unfold as the year progresses. In March 2020, various regulatory agencies, including the Board of Governors of the Federal Reserve System and the Federal Deposit Insurance Corporation, or the agencies, issued an interagency statement on loan modifications and reporting for financial institutions working with customers affected by COVID-19. The interagency statement was effective immediately and impacted accounting for loan modifications. Under Accounting Standards Codification 310-40, “Receivables – Troubled Debt Restructurings by Creditors,” or ASC 310-40, a restructuring of debt constitutes a troubled debt restructuring, or TDR, if the creditor for economic or legal reasons related to the debtor’s financial difficulties, grants a concession to the debtor that it would not otherwise consider. The agencies confirmed with the staff of the Financial Accounting Standards Boards, or FASB, that short-term modifications made on a good faith basis in response to COVID-19 to borrowers who were current prior to any relief, are not to be considered TDRs. This includes short-term (e.g., six months) modifications such as payment deferrals, fee waivers, extensions of repayment terms, or other delays in payment that are insignificant. Borrowers considered current are those that are less than 30 days past due on their contractual payments at the time a modification program is implemented. This interagency guidance has not had a material impact on the Company’s financial statements for disclosure of the impact to date.
Emerging Growth Company	Emerging Growth Company The Company qualifies as an “emerging growth company” under the Jumpstart Our Business Startups Act of 2012, or the JOBS Act, and may take advantage of certain exemptions from various reporting requirements that are applicable to public companies that are not emerging growth companies, including, but not limited to, not being required to comply with the auditor attestation requirements of Section 404 of the Sarbanes-Oxley Act, reduced disclosure obligations regarding executive compensation in periodic reports and proxy statements, and exemptions from the requirements of holding a nonbinding advisory vote on executive compensation and shareholder approval of any golden parachute payments not previously approved. In addition, even if the Company complies with the greater obligations of public companies that are not emerging growth companies, the Company may avail itself of the reduced requirements applicable to emerging growth companies from time to time in the future, so long as the Company is an emerging growth company. The Company will continue to be an emerging growth company until the earliest to occur of: (1) the end of the fiscal year following the fifth anniversary of the date of the first sale of common equity securities under the Company’s Registration Statement on Form S-1, which was declared effective by the SEC on September 12, 2019; (2) the last day of the fiscal year in which the Company has $1.07 billion or more in annual revenues; (3) the date on which the Company is deemed to be a “large accelerated filer” under the Securities Exchange Act of 1934, as amended, or the “Exchange Act”; or (4) the date on which the Company has, during the previous three-year period, issued publicly or privately, more than $1.0 billion in non-convertible debt securities. Section 107 of the JOBS Act provides that an emerging growth company can take advantage of the extended transition period provided in Section 7(a)(2)(B) of the Securities Act of 1933 for complying with new or revised accounting standards. As an emerging growth company, the Company can delay the adoption of certain accounting standards until those standards would otherwise apply to private companies. The Company elected to take advantage of the benefits of this extended transition period.
New Accounting Pronouncements	Adopted Pronouncements In January 2017, the FASB issued ASU No. 2017-04, Simplifying the Test for Goodwill Impairment. This ASU removes Step 2 of the goodwill impairment test, which requires a hypothetical purchase price allocation. Under the amended guidance, a goodwill impairment charge will now be recognized for the amount by which the carrying value of a reporting unit exceeds its estimated fair value, not to exceed the carrying amount of goodwill. For public business entities that are US Securities and Exchange Commission filers, ASU 2017-04, is effective for interim and annual reporting periods beginning after December 15, 2019. The Company adopted ASU 2017-04 effective January 1, 2020, and the new guidance did not have an impact on the Company’s consolidated financial statements. In August 2018, the FASB issued ASU No. 2018‑13, Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement. This ASU eliminates, adds, and modifies certain disclosure requirements for estimated fair value measurements. Among the changes, entities will no longer be required to disclose the amount of and reasons for transfers between Level 1 and Level 2 of the estimated fair value hierarchy, but will be required to disclose the range and weighted-average used to develop significant unobservable inputs for Level 3 estimated fair value measurements. ASU 2018‑13 is effective for all entities interim and annual reporting periods beginning after December 15, 2019. The Company adopted ASU 2018-13 effective January 1, 2020, and the revised disclosure requirements did not have a material impact on the Company’s consolidated financial statements. In August 2018, the FASB issued ASU 2018-15, Intangibles – Goodwill and Other – Internal Use Software (Subtopic 350-40) – Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That is a Service Contract. ASU 2018-15 aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software (and hosting arrangements that include an internal-use software license). The accounting for the service element of a hosting arrangement that is a service contract is not affected by these amendments. ASU 2018-15 was effective for the Company on January 1, 2020, and did not have an impact on the Company’s consolidated financial statements. Pronouncements Not Yet Effective In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. This ASU requires a new impairment model known as the current expected credit loss, or CECL, which significantly changes the way impairment of financial instruments is recognized by requiring immediate recognition of estimated credit losses expected to occur over the remaining life of financial instruments. The main provisions of ASU 2016-13 include (1) replacing the “incurred cost” approach under GAAP with an “expected loss” model for instruments measured at amortized cost, (2) requiring entities to record an allowance for credit losses related to available-for-sale debt securities rather than a direct write-down of the carrying amount of the investments, as is required by the other-than-temporary impairment model under current GAAP, and (3) a simplified accounting model for purchase credit-impaired debt securities and loans. In November 2019, the FASB issued ASU No. 2019-10, Financial Instruments – Credit Losses (Topic 326). This update amends the effective date of ASU No. 2016-13 for certain entities, including private companies and smaller reporting companies, until fiscal years beginning after December 15, 2022, including interim periods within those fiscal periods. Early adoption is permitted. As an emerging growth company, the Company can take advantage of this delay and plans to adopt the standard with the amended effective date. The Company does not plan to early adopt this standard but continues to work on its implementation. The Company continues collecting and retaining loan and credit data and evaluating various loss estimation models. While we currently cannot reasonably estimate the impact of adopting this standard, we expect the impact will be influenced by the composition, characteristics, and quality of our loan portfolio, as well as the general economic conditions and forecasts as of the adoption date. In April 2019, the FASB issued ASU 2019‑04, Codification Improvements to Topic 326, Financial Instruments—Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments, which affects a variety of topics in the Codification and applies to all reporting entities within the scope of the affected accounting guidance. The Company will consider these clarifications and improvements in determining the appropriate adoption of ASU 2019-04. In May 2019, the FASB issued ASU 2019‑05, Targeted Transition Relief to provide entities with an option to irrevocably elect the fair value option applied on an instrument-by-instrument basis for eligible instruments. In November 2019, the FASB Issued ASU 2019-10, which amends the effective date of this ASU for certain entities, including private companies and smaller reporting companies until after December 15, 2022, including interim periods within those fiscal years. As an emerging growth company, the Company can take advantage of this delay and plans to adopt the standard with the amended effective date. This update is not expected to have a significant impact on the Company’s consolidated financial statements. In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740), which simplifies accounting for income taxes by removing certain exceptions to the general principles in Topic 740. The amendments also improve consistent application of and simplify GAAP for the areas of Topic 740 by clarifying and amending existing guidance. This guidance is effective for fiscal years, and interim periods within those fiscal years beginning after December 15, 2020, for public business entities. For private companies and smaller reporting companies, this guidance is effective for fiscal years, and interim periods within those fiscal years beginning after December 15, 2021. As an emerging growth company, the Company can take advantage of this later effective date. Early adoption of the amendments is permitted, including adoption in any interim period for which financial statements have not yet been issued. Depending on the amendment, adoption may be applied on the retrospective, modified retrospective, or prospective basis. The Company is currently reviewing the provisions of this new pronouncement, but does not expect adoption of this guidance to have a material impact on the Company’s consolidated financial statements. In January 2020, the FASB issued ASU 2020-01, Investments – Equity Securities (Topic 321), Investments – Equity Method and Joint Ventures (Topic 323), and Derivatives and Hedging (Topic 815) – Clarifying the Interactions between Topic 321, Topic 323, and Topic 815. The ASU is based on a consensus of the Emerging Issues Task Force and is expected to increase comparability in accounting for these transactions. ASU 2016-01 made targeted improvements to accounting for financial instruments, including providing an entity the ability to measure certain equity securities without a readily determinable fair value at cost, less any impairment, plus or minus changes resulting from observable price changes in orderly transactions for the identical or a similar investment of the same issuer. Among other topics, the amendments clarify that an entity should consider observable transactions that require it to either apply or discontinue the equity method of accounting. For public business entities, the amendments in the ASU are effective for fiscal years beginning after December 15, 2020, and interim periods within those fiscal years. Early adoptions is permitted. The Company does not expect the adoption of ASU 2020-01 to have a material impact on its consolidated financial statements. In March 2020, the FASB issued ASU 2020-03. Codification Improvements to Financial Instruments. This ASU represents changes to clarify or improve the Accounting Standards Codification, or ASC, related to seven topics. The amendments make the ASC easier to understand and easier to apply by eliminating inconsistencies and providing clarifications. Issues 1, 2, 3, 4, and 5 are conforming amendments and for public business entities effective upon the issuance of the standard. Issues 6 and 7 are amendments that affect the guidance in ASU 2016-13. The Company will consider these clarifications and improvements in determining the appropriate adoption of ASU 2016-13. In March 2020, the FASB issued ASU 2020-04, Reference Rate Reform (Topic 848): Facilitation of the Effects of Reference Rate Reform on Financial Reporting. These amendments provide temporary optional guidance to ease the potential burden in accounting for reference rate reform. The ASU provides optional expedients and exceptions for applying GAAP to contract modifications and hedging relationships, subject to meeting certain criteria, that reference LIBOR or another reference rate expected to be discontinued. It is intended to help stakeholders during the global market-wide reference rate transition period. The guidance is effective for all entities as of March 12, 2020 through December 31, 2022. The Company is assessing ASU 2020-04 and its impact on the Company’s transition away from LIBOR for its loan and other financial instruments.