Significant Accounting Policies (Policies)	12 Months Ended
Significant Accounting Policies (Policies)	Dec. 31, 2017
Accounting Policies [Abstract]
Consolidation, Policy [Policy Text Block]	Principles of Consolidation The consolidated financial statements include the accounts of Business First Bancshares, Inc. and its wholly-owned subsidiary, Bus iness First Bank (the Bank), and the Bank’s wholly-owned subsidiary, Business First Insurance, LLC (collectively, the Company). All significant intercompany balances and transactions have been eliminated.
Nature of Operations [Policy Text Block]	Nature of Operations The Bank operates out of branch locations, loan production offices, and one wealth solutions office in markets across Louisiana and Texas. As a state bank, it is subject to regulation by the Office of Financial Institutions, State of Louisiana, and the Federal Deposit Insurance Corporation, and undergoes periodic examinations by these agencies. The Company is also regulated by the Federal Reserve and is subject to periodic examinations. After the close of business on March 31, 2015, the Company merged with American Gateway Financial Corporation (“AGFC”), parent bank holding company for American Gateway Bank, pursuant to which the operations of AGFC were merged with the Company. Prior to the merger, AGFC was a full service bank with 10 branches located in the Baton Rouge metro region. Total assets acquired were $371.5 million, which included loans of $143.2 million and investment securities of $108.4 million, and deposits of $283.3 million were assumed in the transaction. Shareholders of AGFC received merger consideration of $10 in cash and 11.88 shares of Business First Bancshares, Inc. common stock in exchange for each share of AGFC common stock, representing a total consideration of $47.9 million.
Use of Estimates, Policy [Policy Text Block]	Estimates Preparation of financial statements in conformity with U.S. GAAP requ ires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying disclosures. These estimates are based on management’s best knowledge of current events and actions the Company may undertake in the future. Estimates are used in accounting for, among other items, the allowance for loan losses, useful lives for depreciation and amortization, fair value of financial instruments, deferred taxes, and contingencies. Estimates that are particularly susceptible to significant change for the Company include the determination of the allowance for loan losses and the assessment of deferred tax assets and liabilities, and therefore are critical accounting policies. Management does not anticipate any material changes to estimates in the near term. Factors that may cause sensitivity to the aforementioned estimates include but are not limited to: external market factors such as market interest rates and employment rates, changes to operating policies and procedures, economic conditions in our markets, and changes in applicable banking regulations. Actual results may ultimately differ from estimates, although management does not generally believe such differences would materially affect the consolidated financial statements in any individual reporting period presented. The Bank ’s loans are generally secured by specific items of collateral including real property, business assets, and consumer assets. Although the Bank has a diversified loan portfolio, a substantial portion of its debtors’ ability to honor their contracts is dependent on local economic conditions in the Bank’s market area. While management uses available information to recognize losses on loans, further reductions in the carrying amounts of loans may be necessary based on changes in local economic conditions. In addition, regulatory agencies, as an integral part of their examination process, periodically review the estimated losses on loans. Such agencies may require the Bank to recognize additional losses based on their judgments about information available to them at the time of their examination. Because of these factors, it is reasonably possible that the estimated losses on loans may change materially in the near term. However, the amount of the change that is reasonably possible cannot be estimated.
Business Combinations Policy [Policy Text Block]	Acquisition Accounting Acquisitions are accounted for u nder the purchase method of accounting. Purchased assets and assumed liabilities are recorded at their respective acquisition date fair values, and identifiable intangible assets are recorded at fair value. If the consideration given exceeds the fair value of the net assets received, goodwill is recognized. Fair values are subject to refinement for up to one year after the closing date of an acquisition as information relative to closing date fair values becomes available.
Investment, Policy [Policy Text Block]	Securities Management determin es the appropriate classification of debt securities (held to maturity, available for sale or trading) at the time of purchase and re-evaluates this classification periodically. Securities classified as available for sale are those debt securities the Bank intends to hold for an indefinite period of time but not necessarily to maturity. Any decision to sell a security classified as available for sale would be based on various factors, including significant movements in interest rates, changes in the maturity mix of the Bank’s assets and liabilities, liquidity needs, regulatory capital considerations, and other similar factors. Securities available for sale are recorded at fair value. Unrealized gains or losses are reported as a component of comprehensive income. Realized gains or losses, determined on the basis of the cost of specific securities sold, are included in earnings. Securities classified as held to maturity are those debt securities the Bank has both the intent and ability to hold to maturity regardless of changes in market conditions, liquidity needs or changes in general economic conditions. These securities are recorded at cost adjusted for amortization of premium and accretion of discount, computed by various methods approximating the interest method over their contractual lives. The Bank has no securities classified as held to maturity at December 31, 2017 and 2016. Securities classified as trading are those securities held for resale in anticipation of short-term market movements. These securities are recorded at market value with any market adjustments included in earnings. The Bank has no securities classified a s trading at December 31, 2017 and 2016. The Bank has invested in Federal Home Loan Bank (FHLB) stock which is reflected at cost in these financial statements. As a member of the FHLB System, the Bank is required to purchase and maintain stock in an amoun t determined by the FHLB. The FHLB stock is redeemable at par value at the discretion of the FHLB.
Finance, Loans and Leases Receivable, Policy [Policy Text Block]	Loans Loans are stated at principal amounts outstanding less the allowance for loan losses. Interest on commercial and individual loans is accrued daily b ased on the principal outstanding. Generally, the Bank discontinues the accrual of interest income when a loan becomes 90 days past due as to principal or interest. When a loan is placed on nonaccrual status, previously recognized but uncollected intere st is reversed to income or charged to the allowance for loan losses. Subsequent cash receipts on nonaccrual loans are accounted for on the cost recovery method until the loans qualify for return to accrual status. Loans are returned to accrual status when all the principal and interest amounts contractually due are brought current and future payments are reasonably assured. The Bank classifies loans as impaired when it is probable the Bank will be unable to collect the scheduled payments of principal a nd interest when due according to the contractual terms of the loan agreement. The measurement of impaired loans is based on the present value of the expected future cash flows discounted at the loan’s effective interest rate or the loan’s observable market price, or based on the fair value of the collateral if the loan is collateral dependent.
Certain Loans and Debt Securities Acquired in Transfer, Recognizing Interest Income on Impaired Loans, Policy [Policy Text Block]	Acquired Loans Purchased loans acquired in a business combination are recorded at their estimated fair value as of the acquisition date and there is no carryover of the seller ’s allowance for loan losses. The Company accounts for acquired impaired loans in accordance with ASC 310 - 30, Loans and Debt Securities Acquired with Deteriorated Credit Quality (“ASC 310 - 30” ). An acquired loan is considered impaired when there is evidence of credit deterioration since origination and it is probable at the date of acquisition that the Company will be unable to collect all contractually required payments. Purchased credit impaired loans are accounted for individually. The Company estimates the amount and timing of undiscounted expected cash flows for each loan, and the expected cash flows in excess of fair value is recorded as interest income over the remaining life of the loan (accretable yield). The excess of the loan’s contractual principal and interest over the expected cash flows is not recorded (nonaccretable difference). Over the life of the loan, expected cash flows continue to be estimated. If the expected cash flows decrease, a provision for loan losses and the establishment of an allowance for loan losses with respect to the acquired impaired loan is recorded. If the expected cash flows increase, it is recognized as part of future interest income. The performing loans are accounted for under ASC 310 - 20, Nonrefundable Fees and Other Costs (“ASC 310 - 20” ) , with the related discount being adjusted for over the life of the loan and recognized as interest income.
Loans and Leases Receivable, Allowance for Loan Losses Policy [Policy Text Block]	Allowance for Loan Losses The allowance for loan losses is maintained at a level which, in management ’s judgment, is adequate to absorb credit losses inherent in the loan portfolio. The allowance for loan losses is based upon management’s review and evaluation of the loan portfolio. Factors considered in the establishment of the allowance for loan losses include management’s evaluation of specific loans; the level and composition of classified loans; historical loss experience; results of examinations by regulatory agencies; an internal asset review process; expectations of future economic conditions and their impact on particular borrowers; and other judgmental factors. Allowances for impaired loans are generally determined based on collateral values or the present value of estimated cash flows. Management obtains independent appraisals for significant collateral in determining collateral values. Although management uses available information to recognize losses on loans, because of uncertainties associated with local economic conditions, collateral values, and future cash flows on impaired loans, it is reasonably possible that a material change could occur in the allowance for loan losses in the near term. However, the amount of the change that is reasonably possible cannot be estimated. Loans acquired in business combinations are initially recorded at fair value, which includes an estimate of credit losses expected to be realized over the remaining lives of the loans and, therefore, no corresponding allowance for loan losses i s recorded for these loans at acquisition. Methods utilized to estimate any subsequently required allowance for loan losses for acquired loans not deemed credit-impaired at acquisition are similar to originated loans; however, the estimate of loss is based on the unpaid principal balance and then compared to any remaining unaccreted purchase discount. To the extent the calculated loss is greater than the remaining unaccreted discount, an allowance is recorded for such difference. The allowance for loan l osses is based on estimates of potential future losses, and ultimate losses may vary from the current estimates. These estimates are reviewed periodically and as adjustments become necessary, the effect of the change in estimate is charged to operating expenses in the period incurred. All losses are charged to the allowance for loan losses when the loss actually occurs or when management believes that the collectability of the principal is unlikely. Recoveries are credited to the allowance at the time of recovery.
Property, Plant and Equipment, Policy [Policy Text Block]	Premises and Equipment Premises and equipment are stated at cost less accumulated depreciation. Depreciation is provided at rates based upon estimated useful service lives using the straight-line method for financial reporting and accelerated methods for tax reporting purposes. The costs of assets retired or otherwise disposed of and the related accumulated depreciation are eliminated from the accounts in the year of disposal and the resulting gains or losses are included in current operations. Expenditures for maintenance and repairs are charged to operations as incurred. Costs of major additions and improvements are capitalized.
Loans and Leases Receivable, Real Estate Acquired Through Foreclosure, Policy [Policy Text Block]	Other Real Estate Owned Real estate properties acquired through or in lieu of loan foreclosure or negotiated settlement are initially recorded at the fair value less estimated selling cost at the date of acquisition. Any write-downs based on the asset’s fair value at the date of acquisition are charged to the allowance for loan losses. After foreclosure, valuations are periodically performed by management and property held for sale is carried at the lower of the new cost basis or fair value less cost to sell. Impairment losses on property to be held and used are measured as the amount by which the carrying amount of a property exceeds its fair value. Costs of significant property improvements are capitalized, whereas costs relating to holding property are expensed. Valuations are periodically performed by management, and any subsequent write-downs are recorded as a charge to operations, if necessary, to reduce the carrying value of a property to the lower of its cost or fair value less cost to sell. There was no carrying amount of residential real estate included in other real estate owned at December 31, 2017 and 2016.
Goodwill and Intangible Assets, Policy [Policy Text Block]	Goodwill and Other Intangible Assets Goodwill represents the excess of the purchase price over the fair value of the net identifiable assets acquired in a business combination. Goodwill and other intangible assets deemed to have an indefinite useful life are not amortized but instead are su bject to review for impairment annually, or more frequently if deemed necessary. Intangible assets with estimable useful lives are amortized over their respective estimated useful lives and reviewed for impairment. If impaired, the asset is written down to its estimated fair value. Core deposit intangibles representing the value of the acquired core deposit base are generally recorded in connection with business combinations involving banks and branch locations. The Company’s policy is to amortize core deposit intangibles on a straight line basis over their estimated useful life of 10 years. Core deposit intangibles are tested for impairment whenever events or changes in circumstances indicate the carrying amount of the assets may not be recoverable from future undiscounted cash flows.
Income Tax, Policy [Policy Text Block]	Income Taxes The provision for income taxes is based on amounts reported in the statement of income after exclusion of nontaxable income such as interest on state and municipal securities. Also, certain items of income and expense are recognized in different time periods for financial statement purposes than for income tax purposes. Thus, provisions for deferred taxes are recorded in recognition of such temporary differences. Deferred taxes are provided utilizing a lia bility method whereby deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the reported amounts of existing assets and liabilities and their respective tax basis. Deferred tax assets and liabilities are adjusted for the effects of changes in tax laws and rates on the date of enactment. The Company files a consolidated federal income tax return. Consolidated income tax expense is allocated on the basis of each entity ’s income adjusted for permanent differences. The Company evaluates all significant tax positions as required by accounting principles generally accepted in the United States of America. As of December 31, 2017, the Company does not believe it has taken any positions that would require the recording of any additional tax liability, nor does it believe there are any unrealized tax benefits that would either increase or decrease within the next year. The Company files income tax returns in the U.S. federal jurisdiction and the state of Louisiana. With few exceptions, the Company is no longer subject to federal and state income tax examinations by tax authorities for years before 2014. Any interest and penalties assessed by income taxing authorities are not significant, and are included in other expenses in these financial statements, as applicable.
Share-based Compensation, Option and Incentive Plans Policy [Policy Text Block]	Stock Based Compensation As described in Note 15, the Company has issued stock warrants, stock options, stock grants and restricted stock awards which incorporate stock based compensation. The Company has adopted a fair value based method of accounting for these awards. The compensation cost is measured at the grant date based on the value of the award and is recognized over the required service period, which is usually the vesting period.
Cash and Cash Equivalents, Policy [Policy Text Block]	Statements of Cash Flows For purposes of report ing cash flows, cash and cash equivalents include cash on hand and deposits in other financial institutions.
Comprehensive Income, Policy [Policy Text Block]	Comprehensive Income Accounting principles generally require that recognized revenue, expenses, gains and losses be included in net income. Altho ugh certain changes in assets and liabilities, such as unrealized gains and losses on available for sale securities, are reported as a separate component of the equity section of the balance sheet, such items, along with net income, are components of comprehensive income. The components of comprehensive income are disclosed on the Consolidated Statements of Comprehensive Income for all periods presented.
Advertising Cost, Policy, Expensed Advertising Cost [Policy Text Block]	Advertising The Company expenses all costs of advertising and promotion the first time the advertising or promotion takes place. For the years ended December 31, 2017, 2016 and 2015, the Company expensed costs of $762,000, $849,000 and $437,000, respectively.
New Accounting Pronouncements, Policy [Policy Text Block]	Recent Accounting Pronouncements In May 2014, the FASB issued ASU 2014 - 09, Revenue from Contracts with Customers (Topic 606 ) . This ASU implements a common revenue standard and clarifies the principles used for recognizing revenue. The amendments of the ASU clarify that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled to in exchange for those goods or services. This guidance does not apply to revenue associated with financial instruments, including loans and securities that are accounted for under other GAAP, which comprises a significant portion of our revenue stream. The amendments of the ASU will be effective for the Company beginning January 1, 2018. The Company will adopt the amendments beginning January 1, 2018 and does not expect a significant impact to the Company’s consolidated financial statements. In January 2016, the FASB issued ASU No. 2016 - 01, Financial Instruments - Overall (Subtopic 825 - 10 ), Recognition and Measurement of Financial Assets and Financial Liabilities. The provisions of the update require equity investments to be measured at fair value with changes in fair value recognized in net income. However, an entity may choose to measure equity investments that do not have readily determinable fair values at cost minus impairment. The update also simplifies the impairment assessment of equity investments without readily determinable fair values by requiring a qualitative assessment to identify impairment. It also eliminates the requirement to disclose the fair value of financial instruments measured at amortized cost for entities that are not public business entities, and eliminates the requirement for public business entities to disclose the methods and significant assumptions used to estimate the fair value for financial instruments measured at amortized cost on the balance sheet. ASU No. 2016 - 01 requires public business entities to use the exit price notion when measuring the fair value of financial instruments for disclosure purposes. It also requires an entity to present separately in other comprehensive income the portion of the total change in the fair value of a liability resulting from a change in the instrument-specific credit risk when the entity has elected to measure the liability at fair value in accordance with the fair value option for financial instruments. The update requires separate presentation of financial assets and financial liabilities by category and form on the balance sheet or the accompanying notes to the financial statements. In addition, the update clarifies that an entity should evaluate the need for a valuation allowance on a deferred tax asset related to available-for-sale securities in combination with the entity’s other deferred tax assets. For public business entities, the amendments in the update are effective for fiscal years beginning after December 15, 2017, including interim periods. The adoption of this ASU is not expected to have a material impact on the Company’s consolidated financial statements. ASU No. 2016 - 01 will be effective for the Company on January 1, 2018. In February 2016, the FASB issued ASU 2016 - 02, Leases (Topic 842 ), Conforming Amendments Related to Leases . This ASU amends the codification regarding leases in order to increase transparency and comparability. The ASU requires companies to recognize lease assets and liabilities on the statement of condition and disclose key information about leasing arrangements. A lessee would recognize a liability to make lease payments and a right-of-use asset representing its right to use the leased asset for the lease term. The ASU is effective for the Company on January 1, 2019. We expect recorded assets and liabilities to increase upon adoption of the standard as it relates to operating leases in which the Company is the lessee. See Note 20 for a summary of minimum future lease payments under operating leases as of December 31, 2017. In March 2016, the FASB issued ASU 2016 - 09, Compensation - Stock Compensation (Topic 718 ), Improvements to Employee Share-Based Payment Accounting . The ASU amends the codification to simplify several aspects of the accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification in the statement of cash flows. The amendments also allow an accounting policy election to account for forfeitures as they occur. This ASU is effective for annual and interim periods beginning after December 15, 2016, with early adoption permitted. The Company elected an accounting policy to account for forfeitures as they occur upon adoption of this ASU. The adoption of this ASU did not have a material effect on the Company’s consolidated financial statements. In June 2016, the FASB issued ASU 2016 - 13, Financial Instruments – Credit Losses (Topic 326 ), Measurement of Credit Losses on Financial Instruments . The amendments introduce an impairment model that is based on current expected credit losses (“CECL”), rather than incurred losses, to estimate credit losses on certain types of financial instruments (ex. loans and held to maturity securities), including certain off-balance sheet financial instruments (ex. commitments to extend credit and standby letters of credit that are not unconditionally cancellable). The “CECL” should consider historical information, current information, and reasonable and supportable forecasts, including estimates of prepayments, over the contractual term. An entity must use judgment in determining the relevant information and estimation methods that are appropriate in its circumstances. Financial instruments with similar risk characteristics may be grouped together when estimating the “CECL”. The allowance for credit losses for purchased financial assets with a more-than-insignificant amount of credit deterioration since origination that are measured at amortized cost basis is determined in a similar manner to other financial assets measured at amortized cost basis; however, the initial estimate of expected credit loss would be recognized through an allowance for credit losses with an offset (i.e. increase) to the purchase price at acquisition. Only subsequent changes in the allowance for credit losses are recorded as a credit loss expense for these assets. The ASU also amends the current available for sale security impairment model for debt securities whereby credit losses relating to available for sale debt securities should be recorded through an allowance for credit losses. This ASU is effective for the Company on January 1, 2020. The amendments will be applied through a modified retrospective approach, resulting in a cumulative-effect adjustment to retained earnings as of the beginning of the first reporting period in which the guidance is effective. The Company is currently planning for the implementation of this ASU. While we are currently unable to reasonably estimate the impact of adopting ASU 2016 - 13, we expect that the impact of adoption could be significantly influenced by the composition, characteristics and quality of our loan portfolio as well as the prevailing economic conditions and forecasts as of the adoption date. As part of our evaluation process, we have established a working group that includes individuals from various functional areas to assess processes, portfolio segregation, systems requirements and needed resources to implement this new accounting standard. In January 2017, the FASB issued ASU 2017 - 01, Business Combinations (Topic 805 ): Clarifying the Definition of a Business, which introduces amendments intended to clarify the definition of a business with the objective of adding guidance to assist entities with evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. The amendments will be applied prospectively and are effective for annual reporting periods beginning after January 1, 2018, including interim reporting periods within those periods. The adoption of this ASU is not expected to have a significant impact on the Company’s consolidated financial statements. On January 26, 2017, the FASB issued ASU 2017 - 04, Intangibles – Goodwill and Other (Topic 350 ) which simplifies the accounting for goodwill impairment. The guidance in this ASU removes Step 2 of the goodwill impairment test, which requires a hypothetical purchase price allocation. The goodwill impairment will now be the amount by which a reporting unit’s carrying value exceeds its fair value, not to exceed the carrying amount of goodwill. All other goodwill impairment guidance will remain largely unchanged. Entities will continue to have the option to perform a qualitative assessment to determine if a quantitative impairment test is necessary. The same one -step impairment test will be applied to goodwill at all reporting units, even those with zero or negative carrying amounts. Entities will be required to disclose the amount of goodwill at reporting units with zero or negative carrying amounts. The revised guidance will be applied prospectively, and is effective for the Company on January 1, 2020. Early adoption is permitted for any impairment tests performed after January 1, 2017. The adoption of this ASU is not expected to have a material effect on the Company’s consolidated financial statements. In May 2017, the FASB issued ASU 2017 - 09, Compensation – Stock Compensation (Topic 718 ): Scope of Modification Accounting which clarifies when changes to terms or conditions of a share-based payment award must be accounted for as a modification. Under the new guidance, an entity will not apply modification accounting to a share-based payment award if all of the following are the same immediately before and after the change: (i) the fair value of the award, (ii) the vesting conditions of the award, and (iii) the classification of the award as either an equity or liability instrument. ASU 2017 - 09 will be effective for the Company beginning January 1, 2018. The guidance requires companies to apply the requirements prospectively to awards modified on or after the adoption date. The adoption of this ASU is not expected to have a significant impact on the Company’s consolidated financial statements. On February 14, 2018, the FASB issued Accounting Standards Update (ASU) 2018 - 02, Income Statement—Reporting Comprehensive Income (Topic 220 ): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income . The amendments in this ASU address a narrow-scope financial reporting issue related to the tax effects that may become stranded in accumulated other comprehensive income as a result of the Tax Cuts and Jobs Act (TCJA). Under the amendments in ASU 2018 - 02, an entity may elect to reclassify the income tax effects of the TCJA on items within accumulated other comprehensive income to retained earnings. The Company has adopted this ASU effective December 31, 2017. Under the provisions of this ASU, the Company has reclassified the stranded income tax effects associated with TCJA from accumulated other comprehensive income to retained earnings. The stranded income tax effects were associated with unrealized losses on securities available for sale at December 31, 2017.