Significant Accounting Policies (Policies)	9 Months Ended
Significant Accounting Policies (Policies)	Sep. 30, 2019
Significant Accounting Policies
Organization	Organization Alerus Financial Corporation is a financial holding company organized under the laws of the state of Delaware. Alerus Financial Corporation (the “Company”) and its subsidiaries is a diversified financial services company headquartered in Grand Forks, North Dakota. Through its subsidiary, Alerus Financial, National Association (the “Bank”), the Company provides innovative and comprehensive 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 (“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 (“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. 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, 2018, included in the Company’s prospectus filed with the SEC on September 13, 2019, pursuant to Rule 424(b)(4) under the Securities Act of 1933.
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 (“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.
Emerging Growth Company	Emerging Growth Company The Company qualifies as an “emerging growth company” under the Jumpstart Our Business Startups Act of 2012 (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 (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. Management cannot predict if investors will find the Company’s common stock less attractive because it will rely on these exemptions. If some investors find the Company’s common stock less attractive as a result, there may be a less active trading market for its common stock and the Company’s stock price may be more volatile. 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.
Recent Accounting Pronouncements	Adopted Pronouncements In February 2016, the FASB issued ASU No. 2016‑02, “Leases.” Under the new guidance, lessees will be required to recognize the following for all leases (with the exception of short‑term leases): 1) a lease liability, which is the present value of a lessee’s obligation to make lease payments, and 2) a right‑of‑use asset, which is an asset that represents the lessee’s right to use, or control the use of, a specified asset for the lease term. Lessor accounting under the new guidance remains largely unchanged as it is substantially equivalent to existing guidance for sales‑type leases, direct financing leases, and operating leases. Leveraged leases have been eliminated, although lessors can continue to account for existing leveraged leases using the current accounting guidance. Other limited changes were made to align lessor accounting with the lessee accounting model and the new revenue recognition standard. All entities will classify leases to determine how to recognize lease‑related revenue and expense. Quantitative and qualitative disclosures will be required by lessees and lessors to meet the objective of enabling users of financial statements to assess the amount, timing, and uncertainty of cash flows arising from leases. The intention is to require enough information to supplement the amounts recorded in the financial statements so that users can understand more about the nature of an entity’s leasing activities. ASU No. 2016‑02 is effective for interim and annual reporting periods beginning after December 15, 2018. All entities are required to use a modified retrospective approach for leases that exist or are entered into after the beginning of the earliest comparative period in the financial statements. As the Company elected the transition option provided in ASU No. 2018‑11, the modified retrospective approach was applied on January 1, 2019. The Company also elected certain relief options offered in ASU 2016‑02 including the package of practical expedients, the option not to separate lease and non‑lease components and instead to account for them as a single lease component, and the option not to recognize right‑of‑use assets and lease liabilities that arise from short‑term leases (i.e., leases with terms of twelve months or less). The Company did not elect the hindsight practical expedient, which allows entities to use hindsight when determining lease term and impairment of right‑of‑use assets. The Company has several lease agreements, such as branch locations, which are considered operating leases, and therefore, were not previously recognized on the Company’s consolidated balance sheets. The new guidance requires these lease agreements to be recognized on the consolidated balance sheets as a right‑of‑use asset and a corresponding lease liability. The new guidance did not have a material impact on the consolidated statements of income or the consolidated statements of cash flows. See “NOTE 7 Leases” for more information. In March 2017, the FASB issued ASU No. 2017‑08, Premium Amortization on Purchased Callable Debt Securities. This ASU shortens the amortization period for the premium on certain purchased callable debt securities to the earliest call date. Currently, entities generally amortize the premium as an adjustment of yield over the contractual life of the security. ASU 2017‑08 does not change the accounting for purchased callable debt securities held at a discount as the discount will continue to be accreted to maturity. ASU 2017‑08 is effective for public business entities for the interim and annual reporting periods beginning after December 15, 2018. Early adoption is permitted. The guidance calls for a modified retrospective transition approach under which a cumulative‑effect adjustment will be made to retained earnings as of the beginning of the first reporting period in which ASU 2017‑08 is adopted. The Company elected to early adopt this standard as of January 1, 2019, and has evaluated the provisions of ASU 2017‑08 and determined there is no impact on its consolidated financial statements. In August 2017, the FASB issued ASU No. 2017‑12, “Targeted Improvements to Accounting for Hedging Activities.” This ASU’s objectives are to (1) improve the transparency and understandability of information conveyed to financial statement users about an entity’s risk management activities by better aligning the entity’s financial reporting for hedging relationships with those risk management activities; and (2) reduce the complexity of and simplify the application of hedge accounting by preparers. ASU No. 2017‑12 is effective for interim and annual reporting periods beginning after December 15, 2018. The Company currently does not designate any derivative financial instruments as formal hedging relationships, and therefore, does not currently utilize hedge accounting. As such, ASU No. 2017‑12 did not impact the Company’s consolidated financial statements. In September 2018, the FASB issued ASU No. 2018‑07, Compensation (Topic 718): Improvements to Nonemployee Share‑Based Payment Accounting. This ASU has been issued as part of a simplification initiative which will expand the scope of Topic 718 to include share‑based payment transactions for the acquiring of goods and services from non‑employees and expands the scope through the amendments to address and improve aspects of the accounting for non‑employee share‑based payment transactions. The amendments will be effective for public business entities for interim and annual reporting periods beginning after December 15, 2018. The Company elected to early adopt ASU 2018‑07 effective January 1, 2019, and has evaluated the provisions of ASU 2018‑07 and determined there was no significant impact on its consolidated financial statements. Pronouncements Not Yet Effective In September 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 (“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. For public business entities that are U.S. Securities and Exchange Commission filers, ASU 2016‑13 is effective for interim and annual reporting periods beginning after December 15, 2019. For all other public business entities ASU 2016‑13 is effective for interim and annual reporting periods beginning after December 15, 2020. As an emerging growth company, ASU 2016‑13 is effective for interim and annual reporting periods beginning after December 15, 2021, although early adoption is permitted. The Company is currently evaluating the potential impact of ASU 2016‑13 on its consolidated financial statements. On July 17, 2019, the FASB voted to issue a proposal for public comment that would potentially result in a postponement of the required implementation date for ASU 2016‑13, and on October 16, 2019, the FASB approved delaying the implementation of this ASU for private companies and smaller reporting companies until 2023. As an emerging growth company, the Company can take advantage of this delay. Management will continue to monitor any new developments regarding this possible delay. 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 transfer 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. Early adoption is permitted. Entities are also allowed to elect for early adoption of the eliminated or modified disclosure requirements and delay adoption of the new disclosure requirements until their effective date. The revised disclosure requirements will not have a material impact on the Company’s consolidated financial statements. 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. This update is not expected to have a significant impact on the Company’s consolidated financial statements. 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. As an emerging growth company, these ASUs are effective for fiscal years beginning after December 15, 2021, including interim periods within those fiscal years. This update is not expected to have a significant impact on the Company’s consolidated financial statements.