Subordinated Borrowings [Abstract] Schedule of Subordinated Borrowing Schedule of Subordinated Borrowing [Table Text Block] Subordinated Borrowing [Table] Subordinated Borrowing [Table] Business Acquisition [Axis] Business Acquisition [Axis] Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] F&M Bancshares. Inc. F&M Bancshares. Inc. [Member] Represents F&M Bancshares, Inc. Long-term Debt, Type [Axis] Long-term Debt, Type [Axis] Long-term Debt, Type [Domain] Long-term Debt, Type [Domain] Junior Subordinated Debt Junior Subordinated Debt [Member] Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Subordinated Notes Issued 2017 Fixed-to-Floating Rate Subordinated Notes [Member] Fixed-to-Floating Rate Subordinated Notes [Member] Variable Rate [Axis] Variable Rate [Axis] Variable Rate [Domain] Variable Rate [Domain] 3 month LIBOR London Interbank Offered Rate (LIBOR) [Member] Subordinated Borrowing [Line Items] Subordinated Borrowing [Line Items] Debentures, period over which company may defer interest payments abtx_DebenturesPeriodOverWhichTheCompanyMayDeferInterestPayments Represents the period of time over which the Company has the right to defer payment of interest on debentures. Debt instrument, face amount Debt Instrument, Face Amount Borrowed funds Debt Instrument, Fair Value Disclosure Debt instrument, unamortized discount Debt Instrument, Unamortized Discount Junior subordinated debt owed to trusts Junior Subordinated Debenture Owed to Unconsolidated Subsidiary Trust Issuance price, percentage Debt Instrument, Issuance Price, Percentage Debt Instrument, Issuance Price, Percentage Proceeds from issuance of debt Proceeds from Issuance of Debt Stated percentage Debt Instrument, Interest Rate, Stated Percentage Basis spread on variable rate Debt Instrument, Basis Spread on Variable Rate Redemption price, percentage Debt Instrument, Redemption Price, Percentage Investments, Debt and Equity Securities [Abstract] Schedule of Available-for-sale Securities [Table] Schedule of Available-for-sale Securities [Table] Financial Instrument [Axis] Financial Instrument [Axis] Financial Instruments [Domain] Financial Instruments [Domain] U.S. Government and agency securities US Government Agencies Debt Securities [Member] Municipal securities US States and Political Subdivisions Debt Securities [Member] Agency mortgage-backed pass-through securities Agency Mortgage-backed Pass-through Securities [Member] This category includes agency mortgage-backed pass-through securities. Corporate bonds and other Corporate Debt Securities [Member] Debt Securities, Available-for-sale [Line Items] Debt Securities, Available-for-sale [Line Items] Securities in a Continuous Unrealized Loss Position, Less than 12 months, Estimated Fair Value Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months Securities in a Continuous Unrealized Loss Position, Less than 12 Months, Unrealized Losses Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss Securities in a Continuous Unrealized Loss Position, More than 12 Months, Estimated Fair Value Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer Securities in a Continuous Unrealized Loss Position, More than 12 Months Unrealized Losses Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss Securities in a Continuous Unrealized Loss Position, Total Estimated Fair Value Debt Securities, Available-for-sale, Unrealized Loss Position Securities in a Continuous Unrealized Loss Position, Total Unrealized Losses Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss Fair Value Disclosures [Abstract] FAIR VALUE Fair Value Disclosures [Text Block] Income Statement [Abstract] Statement [Table] Statement [Table] Product and Service [Axis] Product and Service [Axis] Product and Service [Domain] Product and Service [Domain] Deposit Account, Nonsufficient Funds Fee Deposit Account, Nonsufficient Funds Fee [Member] Deposit Account, Nonsufficient Funds Fee [Member] Deposit Account, Service Charge Fee Deposit Account, Service Charge Fee [Member] Deposit Account, Service Charge Fee [Member] Statement [Line Items] Statement [Line Items] INTEREST INCOME: INTEREST INCOME: Loans, including fees Interest and Fee Income, Loans and Leases Securities: Securities: Taxable Interest Income, Securities, Operating, Taxable Tax-exempt Interest Income, Securities, Operating, Tax Exempt Deposits in other financial institutions Interest Income, Deposits with Financial Institutions Total interest income Interest and Dividend Income, Operating INTEREST EXPENSE: Interest Expense [Abstract] Demand, money market and savings deposits abtx_InterestExpenseDemandMoneyMarketAndSavingsDeposits Interest expense incurred on demand deposit accounts that represent borrowings rather than outstanding drafts, interest expense incurred on all money market deposits, and interest expense incurred on all savings account deposits. Certificates and other time deposits Interest Expense, Time Deposits Borrowed funds Interest Expense, Borrowings Subordinated debt Interest Expense, Subordinated Notes and Debentures Total interest expense Interest Expense NET INTEREST INCOME Interest Income (Expense), Net Provision for loan losses Provision for Loan and Lease Losses Net interest income after provision for loan losses Interest Income (Expense), after Provision for Loan Loss NONINTEREST INCOME: Noninterest Income [Abstract] Revenue from contract with customer Revenue from Contract with Customer, Including Assessed Tax Gain on sale of other real estate Gains (Losses) on Sales of Other Real Estate Bank owned life insurance income Bank Owned Life Insurance Income Rebate from correspondent bank Rebate from correspondent bank Rebate from correspondent bank Other Noninterest Income, Other Operating Income Total noninterest income Noninterest Income NONINTEREST EXPENSE: Noninterest Expense [Abstract] Salaries and employee benefits Labor and Related Expense Net occupancy and equipment Occupancy, Net Depreciation Depreciation, Nonproduction Data processing and software amortization Information Technology and Data Processing Professional fees Professional Fees Regulatory assessments and FDIC insurance Regulatory assessments and FDIC insurance Amount of expense for regulatory assessments and Federal Deposit Insurance Corporation (FDIC) insurance. Core deposit intangibles amortization Amortization of Intangible Assets Communications Communication Advertising Marketing and Advertising Expense Other Other Noninterest Expense Total noninterest expense Noninterest Expense INCOME BEFORE INCOME TAXES Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Provision for income taxes Income Tax Expense (Benefit) NET INCOME Net Income (Loss) Available to Common Stockholders, Basic EARNINGS PER SHARE: Earnings Per Share [Abstract] Basic (in dollars per share) Earnings Per Share, Basic Diluted (in dollars per share) Earnings Per Share, Diluted Goodwill and Intangible Assets Disclosure [Abstract] Schedule of Intangible Assets and Goodwill Schedule of Intangible Assets and Goodwill [Table Text Block] Schedule of Finite-Lived Intangible Assets, Future Amortization Expense Schedule of Finite-Lived Intangible Assets, Future Amortization Expense [Table Text Block] Deposit Liabilities [Abstract] Deposit Liabilities [Abstract] Schedule of Maturities of Time Deposits Schedule of Maturities of Time Deposits [Table Text Block] Tabular disclosure of time deposits by year of maturity. Goodwill Goodwill [Roll Forward] Balance, beginning Goodwill Balance, ending Schedule of Finite-Lived Intangible Assets [Table] Schedule of Finite-Lived Intangible Assets [Table] Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets, Major Class Name [Domain] Finite-Lived Intangible Assets, Major Class Name [Domain] Core Deposit Intangibles Core Deposits [Member] Finite-Lived Intangible Assets [Line Items] Finite-Lived Intangible Assets [Line Items] Core Deposit Intangibles Finite-lived Intangible Assets [Roll Forward] Balance, beginning Finite-Lived Intangible Assets, Net Amortization Balance, ending Fair Value Measurements, Recurring and Nonrecurring [Table] Fair Value Measurements, Recurring and Nonrecurring [Table] Measurement Basis [Axis] Measurement Basis [Axis] Fair Value Measurement [Domain] Fair Value Measurement [Domain] Portion at Fair Value Measurement [Member] Portion at Fair Value Measurement [Member] Carrying Amount Reported Value Measurement [Member] Estimated Fair Value Estimate of Fair Value Measurement [Member] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] Level 1 Fair Value, Inputs, Level 1 [Member] Level 2 Fair Value, Inputs, Level 2 [Member] Level 3 Fair Value, Inputs, Level 3 [Member] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Cash and cash equivalents Cash and Cash Equivalents, Fair Value Disclosure Available for sale securities Available-for-sale Securities Loans held for investment, net of allowance abtx_LoansHeldForInvestmentNetOfAllowanceFairValue Represents the fair value of loans held for investment (specifically excludes loans held for sale), net of the related allowance. FHLB stock Federal Home Loan Bank Stock Accrued interest receivable abtx_AccruedInterestReceivableFairValue The fair value of accrued interest receivable. Deposits Deposits, Fair Value Disclosure Accrued interest payable abtx_AccruedInterestPayableFairValue The fair value of accrued interest payable. Subordinated debt and subordinated debentures Subordinated Debt Obligations, Fair Value Disclosure GOODWILL AND CORE DEPOSIT INTANGIBLE ASSETS Goodwill and Intangible Assets Disclosure [Text Block] Loans and Allowance for Loan Losses [Abstract] Loans and Allowance for Loan Losses [Abstract] Schedule of Accounts, Notes, Loans and Financing Receivable [Table] Schedule of Accounts, Notes, Loans and Financing Receivable [Table] Financing Receivable Portfolio Segment [Axis] Financing Receivable Portfolio Segment [Axis] Financing Receivable Portfolio Segment [Domain] Financing Receivable Portfolio Segment [Domain] Commercial and industrial Commercial Portfolio Segment [Member] Mortgage warehouse Mortgage Warehouse Portfolio Segment [Member] Portfolio segment of the company's total financing receivables related to commercial receivables. Real estate Real Estate Porfolio Segment [Member] Portfolio segment of the company's total financing receivables related to commercial real estate. Consumer and other Consumer Portfolio Segment [Member] Class of Financing Receivable, Type [Axis] Class of Financing Receivable, Type [Axis] Class of Financing Receivable, Type [Domain] Class of Financing Receivable, Type [Domain] Commercial real estate (including multi-family residential) Commercial Real Estate Including Multi-family Residential but Not Construction and Land Development [Member] Represents commercial real estate including multi-family residential but not construction and land development. Commercial real estate construction and land development Commercial Real Estate Construction and Land Development [Member] Information pertaining to commercial real estate construction and land development. 1-4 family residential (including home equity) 1-4 Family Residential Including Home Equity [Member] Represents real estate for 1-4 family residential including home equity. Residential construction Residential Construction [Member] Represents information pertaining to residential construction. Accounts, Notes, Loans and Financing Receivable [Line Items] Accounts, Notes, Loans and Financing Receivable [Line Items] Number of Contracts Financing Receivable, Modifications, Number of Contracts Pre- Modification of Outstanding Recorded Investment Financing Receivable, Modifications, Pre-Modification Recorded Investment Post- Modification of Outstanding Recorded Investment Financing Receivable, Modifications, Post-Modification Recorded Investment DEPOSITS Deposit Liabilities Disclosures [Text Block] Disclosure of Compensation Related Costs, Share-based Payments [Abstract] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Award Type [Axis] Award Type [Axis] Equity Award [Domain] Equity Award [Domain] Restricted Stock Restricted Stock [Member] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Number of Shares Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] Nonvested share awards outstanding, beginning balance (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number Share awards granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period Share awards vested (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period Unvested share awards forfeited (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period Nonvested share awards outstanding, ending balance (in shares) Weighted Average Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract] Nonvested share awards outstanding, beginning balance (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value Share awards granted (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value Share awards vested (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value Unvested share awards forfeited (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value Nonvested share awards outstanding, ending balance (in dollars per share) Banking and Thrift [Abstract] Schedule of Compliance with Regulatory Capital Requirements under Banking Regulations Schedule of Compliance with Regulatory Capital Requirements under Banking Regulations [Table Text Block] Remaining 2018 Finite-Lived Intangible Assets, Amortization Expense, Remainder of Fiscal Year 2019 Finite-Lived Intangible Assets, Amortization Expense, Year Two 2020 Finite-Lived Intangible Assets, Amortization Expense, Year Three 2021 Finite-Lived Intangible Assets, Amortization Expense, Year Four 2022 Finite-Lived Intangible Assets, Amortization Expense, Year Five Thereafter Finite-Lived Intangible Assets, Amortization Expense, after Year Five Total Schedule of Accounts, Notes, Loans and Financing Receivable Schedule of Accounts, Notes, Loans and Financing Receivable [Table Text Block] Past Due Financing Receivables Past Due Financing Receivables [Table Text Block] Impaired Financing Receivables Impaired Financing Receivables [Table Text Block] Financing Receivable Credit Quality Indicators Financing Receivable Credit Quality Indicators [Table Text Block] Allowance for Credit Losses on Financing Receivables Allowance for Credit Losses on Financing Receivables [Table Text Block] Troubled Debt Restructurings on Financing Receivables Troubled Debt Restructurings on Financing Receivables [Table Text Block] Commitments and Contingencies Disclosure [Abstract] Schedule of Fair Value, Off-balance Sheet Risks [Table] Schedule of Fair Value, Off-balance Sheet Risks [Table] Commitments to extend credit Commitments to Extend Credit [Member] Standby letters of credit Standby Letters of Credit [Member] An irrevocable undertaking (typically by a financial institution) to guarantee payment of a specified financial obligation. Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items] Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items] Fixed Rate Off-balance sheet financial instrument, fixed rate, contractual amount The contractual amount of fixed-rate financial liabilities, which are not recognized in the financial statements (off-balance-sheet) because they fail to meet some other criterion for recognition. Variable Rate Off-balance sheet financial instrument, variable rate, contractual amount The contractual amount of variable-rate financial liabilities, which are not recognized in the financial statements (off-balance-sheet) because they fail to meet some other criterion for recognition. Related Party [Axis] Related Party [Axis] Related Party [Domain] Related Party [Domain] Farmers & Merchants Capital Trust II Farmers & Merchants Capital Trust II [Member] Represents information pertaining to Farmers & Merchants Capital Trust II. Farmers & Merchants Capital Trust III Farmers & Merchants Capital Trust III [Member] Represents information pertaining to Farmers & Merchants Capital Trust III. Issuance Date Debt Instrument, Issuance Date Trust Preferred Securities Outstanding Interest Rate Junior Subordinated Debt Owed to Trusts Maturity Date Debt Instrument, Maturity Date Interest rate, period end Debt Instrument, Interest Rate, Effective Percentage Measurement Frequency [Axis] Measurement Frequency [Axis] Fair Value, Measurement Frequency [Domain] Fair Value, Measurement Frequency [Domain] Recurring Basis Fair Value, Measurements, Recurring [Member] Available for sale securities Debt Securities, Available-for-sale Business Combinations [Abstract] ACQUISITION Business Combination Disclosure [Text Block] EARNINGS PER COMMON SHARE Earnings Per Share [Text Block] Schedule of Business Acquisitions, by Acquisition [Table] Schedule of Business Acquisitions, by Acquisition [Table] Geographical [Axis] Geographical [Axis] Geographical [Domain] Geographical [Domain] Houston, Texas Houston, Texas [Member] Represents Houston, Texas. Beaumont, TX Beaumont, TX [Member] Beaumont, TX [Member] Post Oak Bancshares, Inc. Post Oak Bancshares, Inc. [Member] Post Oak Bancshares, Inc. [Member] Business Acquisition [Line Items] Business Acquisition [Line Items] Number of branches abtx_NumberOfBranches The number of branch offices operated by the entity. Number of shares per common share or option to purchase common shares (in shares) Business Acquisition, Equity Interest Issued Or Issuable, Number Of Shares Per Common Share Or Option To Purchase Common Shares Business Acquisition, Equity Interest Issued Or Issuable, Number Of Shares Per Common Share Or Option To Purchase Common Shares Debt Disclosure [Abstract] Remaining 2018 Long-term Debt, Maturities, Repayments of Principal, Remainder of Fiscal Year 2019 Long-term Debt, Maturities, Repayments of Principal in Year Two 2020 Long-term Debt, Maturities, Repayments of Principal in Year Three 2021 Long-term Debt, Maturities, Repayments of Principal in Year Four Total Long-term Debt Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Federal Home Loan Bank Branch [Axis] Federal Home Loan Bank Branch [Axis] Federal Home Loan Bank Branch [Domain] Federal Home Loan Bank Branch [Domain] Federal Home Loan Bank of Dallas Federal Home Loan Bank of Dallas [Member] Short-term Debt Short-term Debt [Member] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Weighted Average Weighted Average [Member] Expire in September 2018 Expire in September 2018 [Member] Expire in September 2018 [Member] Expire in October 2018 Expire in October 2018 [Member] Expire in October 2018 [Member] Expire in December 2018 Expire in December 2018 [Member] Expire in December 2018 [Member] Expire in January 2019 Expire in January 2019 [Member] Expire in January 2019 [Member] Expire in February 2019 Expire in February 2019 [Member] Expire in February 2019 [Member] Expire in July 2019 Expire in July 2019 [Member] Expire in July 2019 [Member] Expire in August 2019 Expire in August 2019 [Member] Expire in August 2019 [Member] Expire in December 2019 Expire in December 2019 [Member] Expire in December 2019 [Member] Expire in January 2020 Expire in January 2020 [Member] Expire in January 2020 [Member] Credit Facility With Another Financial Institution Credit Facility With Another Financial Institution [Member] Credit Facility With Another Financial Institution [Member] Borrowing Agreement Borrowing Agreement [Member] Represents the borrowing agreement with another financial institution. Prime Rate Prime Rate [Member] Debt Instrument [Line Items] Debt Instrument [Line Items] Maximum borrowing capacity Line of Credit Facility, Maximum Borrowing Capacity Remaining borrowing capacity Line of Credit Facility, Remaining Borrowing Capacity Outstanding amount Long-term Line of Credit Federal Home Loan Bank, advances Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances Federal Home Loan Bank, advances, interest rate Federal Home Loan Bank, Advances, Branch of FHLB Bank, Interest Rate Letters of credit, outstanding amount Letters of Credit Outstanding, Amount Increase (decrease) in other borrowings abtx_IncreaseDecreaseInOtherBorrowings The increase (decrease) during the reporting period in other miscellaneous borrowings not otherwise defined in the taxonomy. Annual reduction in borrowing capacity Line of Credit Facility, Annual Reduction In Borrowing Capacity Line of Credit Facility, Annual Reduction In Borrowing Capacity Repayments of debt Repayments of Debt Interest rate, effective percentage Statement of Cash Flows [Abstract] CASH FLOWS FROM OPERATING ACTIVITIES: Net Cash Provided by (Used in) Operating Activities [Abstract] Net income Net Income (Loss) Attributable to Parent Adjustments to reconcile net income to net cash provided by operating activities: Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Depreciation and core deposit intangibles amortization Depreciation, Depletion and Amortization, Nonproduction Provision for loan losses Provision for Loan, Lease, and Other Losses Excess tax benefit related to the exercise of stock options Excess Tax Benefit from Share-based Compensation, Operating Activities Net amortization of premium on investments Accretion (Amortization) of Discounts and Premiums, Investments Bank owned life insurance Net accretion of discount on loans abtx_AccretionAmortizationOfDiscountsAndPremiumsLoans Represents the net accretion or amortization of a discount or premium on loans. As a noncash item, this element is an adjustment to net income when calculating cash provided by or used in operations using the indirect method. Net amortization of discount on subordinated debentures abtx_AccretionAmortizationOfDiscountsAndPremiumsSubordinatedDebentures Represents the net amortization of premium on subordinated debentures. As a noncash item, this element is an adjustment to net income when calculating cash provided by or used in operations using the indirect method. Net amortization of discount on certificates of deposit abtx_AccretionAmortizationOfdiscountsAndPremiumsCertificatesOfDeposit Represents the net amortization of premium on certificates of deposit. As a noncash item, this element is an adjustment to net income when calculating cash provided by or used in operations using the indirect method. Net gain on sale of other real estate Federal Home Loan Bank stock dividends abtx_FederalHomeLoanBankStockDividends The amount of dividends paid to the company by the Federal Home Loan Bank. Stock based compensation expense Share-based Compensation Increase in accrued interest receivable and other assets abtx_IncreaseDecreaseInAccruedInterestReceivableAndOtherAssets The (increase) decrease during the reporting period in the amount due from borrowers for interest payments, as well as the increase (decrease) during the reporting period in other assets used in operating activities not separately disclosed in the statement of cash flows. Increase in accrued interest payable and other liabilities Increase (Decrease) in Accrued Liabilities and Other Operating Liabilities Net cash provided by operating activities Net Cash Provided by (Used in) Operating Activities CASH FLOWS FROM INVESTING ACTIVITIES: Net Cash Provided by (Used in) Investing Activities [Abstract] Proceeds from maturities and principal paydowns of available for sale securities Proceeds from Maturities, Prepayments and Calls of Debt Securities, Available-for-sale Proceeds from sales of available for sale securities Proceeds from Sale of Debt Securities, Available-for-sale Purchase of available for sale securities Payments to Acquire Debt Securities, Available-for-sale Net change in total loans Payments for (Proceeds from) Loans and Leases Purchase of bank premises and equipment Payments to Acquire Property, Plant, and Equipment Net purchases of Federal Home Loan Bank stock Payments for (Proceeds from) Federal Home Loan Bank Stock Net cash used in investing activities Net Cash Provided by (Used in) Investing Activities CASH FLOWS FROM FINANCING ACTIVITIES: Net Cash Provided by (Used in) Financing Activities [Abstract] Net increase in noninterest-bearing deposits Net Change Noninterest-bearing Deposits, Domestic Net increase in interest-bearing deposits Net Change Interest-bearing Deposits, Domestic Proceeds from borrowed funds Proceeds from Issuance of Long-term Debt Paydowns on borrowed funds Proceeds from the issuance of common stock, stock option exercises, restricted stock awards and the ESPP Proceeds from Issuance of Shares under Incentive and Share-based Compensation Plans, Including Stock Options Net cash provided by financing activities Net Cash Provided by (Used in) Financing Activities NET CHANGE IN CASH AND CASH EQUIVALENTS Cash and Cash Equivalents, Period Increase (Decrease) CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD Cash and Cash Equivalents, at Carrying Value CASH AND CASH EQUIVALENTS, END OF PERIOD SUPPLEMENTAL INFORMATION: Supplemental Cash Flow Information [Abstract] Income taxes paid Income Taxes Paid Interest paid Interest Paid, Including Capitalized Interest, Operating and Investing Activities Debt Securities, Available-for-sale [Table] Debt Securities, Available-for-sale [Table] Security owned and pledged as collateral Security Owned and Pledged as Collateral, Fair Value Minimum Minimum [Member] Maximum Maximum [Member] Commitments to make loans, period abtx_CommitmentsToMakeLoansPeriod The period of time for which commitments to make loans are generally made. Such loans tend to be made for this duration or less. Off-balance-sheet fixed rate loan commitments, interest rate Off Balance Sheet Fixed Rate Loan Commitments, Interest Rate Represents the interest rate for off-balance-sheet fixed rate loan commitments. Off-balance-sheet fixed rate loan commitments, weighted average maturity Off Balance Sheet Fixed Rate Loan Commitments, Weighted Average Maturity Represents the weighted average maturity for off-balance-sheet fixed rate loan commitments. Fair Value, by Balance Sheet Grouping Fair Value, by Balance Sheet Grouping [Table Text Block] Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Fair Value Measurements, Nonrecurring Fair Value Measurements, Nonrecurring [Table Text Block] Net income attributable to common shareholders Basic: Earnings Per Share, Basic [Abstract] Weighted average common shares outstanding (in shares) Weighted Average Number of Shares Outstanding, Basic Weighted average common shares outstanding (in dollars per share) Diluted: Earnings Per Share, Diluted [Abstract] Add incremental shares for: Incremental Weighted Average Shares Attributable to Dilutive Effect [Abstract] Dilutive effect of stock option exercises (in shares) Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements Total (in shares) Weighted Average Number of Shares Outstanding, Diluted Total (in dollars per share) Schedule of Fair Value, Off-balance Sheet Risks Schedule of Fair Value, Off-balance Sheet Risks [Table Text Block] Recorded investment Financing Receivable, Modifications, Recorded Investment Change in method of calculating impairment Allowance for Credit Losses, Change in Method of Calculating Impairment Troubled debt restructuring, write-down Financing Receivables, Impaired, Troubled Debt Restructuring, Write-down Troubled debt restructuring, addition Financing Receivables, Troubled Debt Restructuring, Addition Reflects the amount that add to a troubled debt restructuring during the year. Troubled debt restructuring, addition, outstanding Financing Receivables, Troubled Debt Restructuring, Addition, Outstanding The outstanding amount of the financing receivable that reclassified to new troubled debt restructuring during the year. Nonrecurring Basis Fair Value, Measurements, Nonrecurring [Member] Commercial real estate (including multi-family residential) Commercial Real Estate Portfolio Segment [Member] Impaired loans: Impaired Loans Receivable, Fair Value This element represents the amount of impaired loans receivable reported on the balance sheet at period end measured at fair value. Other real estate owned Other Real Estate Owned, Fair Value The fair value portion of land and buildings obtained as of the balance sheet date of land and buildings. Assets and liabilities are measured at fair value, nonrecurring basis Assets, Fair Value Disclosure STOCK BASED COMPENSATION Disclosure of Compensation Related Costs, Share-based Payments [Text Block] Accounting Policies [Abstract] NATURE OF OPERATIONS AND SUMMARY OF SIGNIFICANT ACCOUNTING AND REPORTING POLICIES Basis of Presentation and Significant Accounting Policies [Text Block] Statement of Financial Position [Abstract] ASSETS Assets [Abstract] Cash and due from banks Cash and Due from Banks Interest-bearing deposits at other financial institutions Interest-bearing Deposits in Banks and Other Financial Institutions Total cash and cash equivalents Available for sale securities, at fair value Loans held for investment Loans and Leases Receivable, Gross Less: allowance for loan losses Loans and Leases Receivable, Allowance Loans, net Loans and Leases Receivable, Net Amount Accrued interest receivable Interest Receivable Premises and equipment, net Property, Plant and Equipment, Net Other real estate owned Real Estate Acquired Through Foreclosure Federal Home Loan Bank stock Bank owned life insurance Bank Owned Life Insurance Goodwill Core deposit intangibles, net Other assets Other Assets TOTAL ASSETS Assets LIABILITIES AND SHAREHOLDERS’ EQUITY Liabilities and Equity [Abstract] LIABILITIES: Liabilities [Abstract] Deposits: Deposits [Abstract] Noninterest-bearing Noninterest-bearing Deposit Liabilities Interest-bearing Interest-bearing Deposit Liabilities [Abstract] Demand Demand Deposit Accounts Money market and savings Deposits, Savings Deposits Certificates and other time Time Deposits Total interest-bearing deposits Interest-bearing Deposit Liabilities Total deposits Deposits Accrued interest payable Interest Payable Borrowed funds Debt, Long-term and Short-term, Combined Amount Subordinated debt Subordinated Debt Other liabilities Other Liabilities Total liabilities Liabilities COMMITMENTS AND CONTINGENCIES (See Note 11) Commitments and Contingencies SHAREHOLDERS’ EQUITY: Stockholders' Equity Attributable to Parent [Abstract] Preferred stock, $1 par value; 1,000,000 shares authorized; no shares issued or outstanding Preferred Stock, Value, Issued Common stock, $1 par value; 40,000,000 shares authorized; 13,340,919 shares issued and outstanding at June 30, 2018 and 13,226,826 shares issued and outstanding at December 31, 2017 Common Stock, Value, Issued Capital surplus Additional Paid in Capital Retained earnings Retained Earnings (Accumulated Deficit) Accumulated other comprehensive (loss) income Accumulated Other Comprehensive Income (Loss), Net of Tax Total shareholders’ equity Stockholders' Equity Attributable to Parent TOTAL LIABILITIES AND SHAREHOLDERS’ EQUITY Liabilities and Equity Schedule of Share-based Compensation, Stock Options, Activity Share-based Compensation, Stock Options, Activity [Table Text Block] Schedule of Share-based Compensation, Restricted Stock Units Award Activity Schedule of Share-based Compensation, Restricted Stock Units Award Activity [Table Text Block] Within one year Time Deposit Maturities, Next Twelve Months After one but within two years Time Deposit Maturities, Year Two After two but within three years Time Deposit Maturities, Year Three After three but within four years Time Deposit Maturities, Year Four After four but within five years Time Deposit Maturities, Year Five Total Number of shares authorized (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized Share-based compensation expense Allocated Share-based Compensation Expense Employee Stock Option Employee Stock Option [Member] Option, cumulative options granted since inception (in shares) abtx_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionCumulativeOptionsGrantedSinceInception Represents the total amount of share-base options granted since the inception of the arrangement. Expiration period Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period Award vesting period Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period Equity instruments other than options, grants in period (in shares) Stock Issued During Period, Shares, New Issues SUBORDINATED DEBT Subordinated Borrowings Disclosure [Text Block] Schedule of Compliance with Regulatory Capital Requirements under Banking Regulations [Table] Schedule of Compliance with Regulatory Capital Requirements under Banking Regulations [Table] Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] Allegiance Bank Allegiance Bank [Member] Represent information pertaining to Allegiance Bank, the Company's subsidiary. Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] Total Capital Total Capital Capital Capital Capital to Risk Weighted Assets Capital to Risk Weighted Assets Capital Required for Capital Adequacy Capital Required for Capital Adequacy Capital Required for Capital Adequacy to Risk Weighted Assets Capital Required for Capital Adequacy to Risk Weighted Assets Plus Capital Conservation Buffer Excess Capital Plus Capital Conservation Buffer to Risk Weighted Assets Excess Capital to Risk Weighted Assets Capital Required to be Well Capitalized Capital Required to be Well Capitalized Capital Required to be Well Capitalized to Risk Weighted Assets Capital Required to be Well Capitalized to Risk Weighted Assets Common Equity Tier 1 Capital Common Equity Tier 1 Capital Common Equity Tier 1 Risk Based Capital abtx_CommonEquityTierOneRiskBasedCapital Common Equity Tier 1 Leverage Capital as defined in the regulations. Common Equity Tier 1 Risk Based Capital to Risk Weighted Assets abtx_CommonEquityTierOneRiskBasedCapitalToRiskWeightedAssets Common Equity Tier 1 capital divided by risk weighted assets as defined by regulations. Common Equity Tier 1 Risk Based Capital Required for Capital Adequacy abtx_CommonEquityTierOneRiskBasedCapitalRequiredForCapitalAdequacy The minimum amount of Common Equity Tier 1 Risk Based Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action. Common Equity Tier 1 Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets Common Equity Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets The minimum amount of Common Equity Tier 1 Risk Based Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action. Plus Common Equity Tier 1 Risk Based Capital Excess Common Equity Tier One Risk Based Capital Excess Common Equity Tier One Risk Based Capital Plus Common Equity Tier 1 Risk Based Capital to Risk Weighted Assets Excess Common Equity Tier One Risk Based Capital to Risk Weighted Assets Excess Common Equity Tier One Risk Based Capital to Risk Weighted Assets Common Equity Tier 1 Risk Based Capital Required to be Well Capitalized abtx_CommonEquityTierOneRiskBasedCapitalRequiredToBeWellCapitalized The amount of Common Equity Tier 1 Risk Based Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action. Common Equity Tier 1 Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets abtx_CommonEquityTierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets The common equity Tier 1 capital ratio (Tier 1 common equity capital divided by risk weighted assets) required to be categorized as "well capitalized" under the regulatory framework for prompt corrective action. Tier 1 Capital Tier I Capital Tier 1 Risk Based Capital Tier One Risk Based Capital Tier 1 Risk Based Capital to Risk Weighted Assets Tier One Risk Based Capital to Risk Weighted Assets Tier 1 Risk Based Capital Required for Capital Adequacy Tier One Risk Based Capital Required for Capital Adequacy Tier 1 Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets Plus Tier 1 Risk Based Capital Excess Tier One Risk Based Capital Plus Tier 1 Risk Based Capital to Risk Weighted Assets Excess Tier One Risk Based Capital to Risk Weighted Assets Tier 1 Risk Based Capital Required to be Well Capitalized Tier One Risk Based Capital Required to be Well Capitalized Tier 1 Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets Tier 1 Capital TierICapitalAbstract0 Tier 1 Leverage Capital Tier One Leverage Capital Tier 1 Leverage Capital to Average Assets Tier One Leverage Capital to Average Assets Tier 1 Leverage Capital Required for Capital Adequacy Tier One Leverage Capital Required for Capital Adequacy Tier 1 Leverage Capital Required for Capital Adequacy to Average Assets Tier One Leverage Capital Required for Capital Adequacy to Average Assets Plus Tier 1 Leverage Capital Excess Tier One Leverage Capital Plus Tier 1 Leverage Capital to Average Assets Excess Tier One Leverage Capital to Average Assets Tier 1 Leverage Capital Required to be Well Capitalized Tier One Leverage Capital Required to be Well Capitalized Tier 1 Leverage Capital Required to be Well Capitalized to Average Assets Tier One Leverage Capital Required to be Well Capitalized to Average Assets Number of offices in which entity operates Number of Offices in which Entity Operates Number of Offices in which Entity Operates Number of loan production offices in which entity operates Number of Loan Production Offices in which Entity Operates Number of Loan Production Offices in which Entity Operates New Accounting Pronouncement, Early Adoption [Table] New Accounting Pronouncement, Early Adoption [Table] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Retained Earnings Retained Earnings [Member] New Accounting Pronouncement, Early Adoption [Line Items] New Accounting Pronouncement, Early Adoption [Line Items] Cumulative effect of new accounting principle in period of adoption Tax Cuts and Jobs Act of 2017, Reclassification from AOCI to Retained Earnings, Tax Effect Tax Cuts and Jobs Act of 2017, Reclassification from AOCI to Retained Earnings, Tax Effect With no related allowance recorded: With no related allowance recorded: Recorded investment with no related allowance recorded Impaired Financing Receivable, with No Related Allowance, Recorded Investment Unpaid principal balance with no related allowance recorded Impaired Financing Receivable, with No Related Allowance, Unpaid Principal Balance With an allowance recorded: With an allowance recorded: Impaired financing receivable, with related allowance, recorded investment Impaired Financing Receivable, with Related Allowance, Recorded Investment Impaired financing receivable, with related allowance, unpaid principal balance Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance Related Allowance Impaired Financing Receivable, Related Allowance Total: Total: Recorded Investment Impaired Financing Receivable, Recorded Investment Unpaid Principal Balance Impaired Financing Receivable, Unpaid Principal Balance Time deposits 250,000 or more abtx_TimeDeposits250000OrMore The aggregate amount of domestic and foreign time deposit liabilities (including certificates of deposits, individual retirement accounts and open accounts) in denominations of $250,000 or more. Brokered deposits Interest-bearing Domestic Deposit, Brokered Reciprocal deposits Interest-bearing Domestic Deposit, Certificates of Deposits BORROWINGS AND BORROWING CAPACITY Debt Disclosure [Text Block] Allowance for loan losses Allowance for loan losses: Allowance for loan losses [Roll Forward] Allowance for loan losses Roll Forward Balance, beginning of period Charge-offs Allowance for Loan and Lease Losses, Write-offs Recoveries Allowance for Loan and Lease Loss, Recovery of Bad Debts Net charge-offs Allowance for Loan and Lease Losses Write-offs, Net Balance, end of period Allowance for loan losses related to: Allowance for loan losses related to: Individually evaluated for impairment Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment Collectively evaluated for impairment Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment Total allowance for loan losses Recorded investment in loans: Recorded investment in loans: Individually evaluated for impairment Financing Receivable, Individually Evaluated for Impairment Collectively evaluated for impairment Financing Receivable, Collectively Evaluated for Impairment Total loans evaluated for impairment LOANS AND ALLOWANCE FOR LOAN LOSSES Loans, Notes, Trade and Other Receivables Disclosure [Text Block] Number of Options Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] Options outstanding, beginning balance (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number Options granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross Options exercised (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period Options forfeited (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures in Period Options outstanding, ending balance (in shares) Number of options vested and exercisable (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number Weighted Average Exercise Price Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract] Options outstanding, beginning balance (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price Options granted (in dollars per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price Options exercised (in dollars per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price Options forfeited (in dollars per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Forfeitures in Period, Weighted Average Exercise Price Options outstanding, ending balance (in dollars per share) Weighted Average Exercise Price, Options vested and exercisable (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price Weighted Average Remaining Contractual Term, Options outstanding Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term Weighted Average Remaining Contractual Term, Options vested and exercisable Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term Aggregate Intrinsic Value, Options outstanding Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value Aggregate Intrinsic Value, Options vested and exercisable Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value Basis of Presentation Basis of Accounting, Policy [Policy Text Block] New Accounting Standards New Accounting Pronouncements, Policy [Policy Text Block] Document And Entity Information Entity Registrant Name Entity Registrant Name Entity Central Index Key Entity Central Index Key Trading Symbol Trading Symbol Current Fiscal Year End Date Current Fiscal Year End Date Entity Filer Category Entity Filer Category Document Type Document Type Document Period End Date Document Period End Date Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Amendment Flag Amendment Flag Entity Common Stock, Shares Outstanding (in shares) Entity Common Stock, Shares Outstanding Available-for-sale Securities Debt Securities, Available-for-sale [Table Text Block] Investments Classified by Contractual Maturity Date Investments Classified by Contractual Maturity Date [Table Text Block] Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value Debt Securities, Available-for-sale, Unrealized Loss Position, Fair Value [Table Text Block] Income Tax Disclosure [Abstract] INCOME TAXES Income Tax Disclosure [Text Block] Financing Receivables, Period Past Due [Axis] Financing Receivables, Period Past Due [Axis] Financing Receivables, Period Past Due [Domain] Financing Receivables, Period Past Due [Domain] Financing Receivables, 30 to 89 Days Past Due Financing Receivables, 30 to 89 Days Past Due [Member] Represents financing receivables that are 30 to 89 days past due. Financing Receivables, Equal to Greater than 90 Days Past Due Financing Receivables, Equal to Greater than 90 Days Past Due [Member] Loans Past Due and Still Accruing Financing Receivable, Recorded Investment, Past Due Nonaccrual Loans Financing Receivable, Recorded Investment, Nonaccrual Status Current Loans Financing Receivable, Recorded Investment, Current Internal Credit Assessment [Axis] Internal Credit Assessment [Axis] Internal Credit Assessment [Domain] Internal Credit Assessment [Domain] Pass Pass [Member] Watch Watch [Member] A category of financing receivables not considered to be special mention, substandard, doubtful, and loss receivables. Special Mention Special Mention [Member] Substandard Substandard [Member] Doubtful Doubtful [Member] OFF-BALANCE SHEET ARRANGEMENTS, COMMITMENTS AND CONTINGENCIES Off Balance Sheet Arrangements Commitments And Contingencies Disclosure [Text Block] The entire disclosure for off-balance sheet arrangements, commitments and contingencies. SECURITIES Investments in Debt and Marketable Equity Securities (and Certain Trading Assets) Disclosure [Text Block] Statement of Comprehensive Income [Abstract] Other comprehensive (loss) income, before tax: Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent [Abstract] Unrealized (loss) gain on securities: Unrealized gain (loss) on securities: Change in unrealized holding (loss) gain on available for sale securities during the period Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, before Tax Total other comprehensive (loss) income Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent Deferred tax benefit (expense) related to other comprehensive (loss) income Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent Other comprehensive (loss) income, net of tax Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent Comprehensive income Comprehensive Income (Loss), Net of Tax, Attributable to Parent Amortized Cost Available-for-sale Equity Securities, Amortized Cost Basis Gross Unrealized Gains Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax Gross Unrealized Losses Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax Fair Value REGULATORY CAPITAL MATTERS Regulatory Capital Requirements under Banking Regulations [Text Block] Capital conservation buffer, phase in amount abtx_CapitalConservationBufferPhaseInAmount Amount resulting from the phase-in of a capital conservation buffer. Capital conservation buffer, increase per year Capital Conservation Buffer, Increase per Year Capital Conservation Buffer, Increase per Year Scenario [Axis] Scenario [Axis] Scenario, Unspecified [Domain] Scenario, Unspecified [Domain] Forecast Scenario, Forecast [Member] Capital conservation buffer abtx_CapitalConservationBuffer An additional amount held in risk-based capital designed to ensure that banks build up capital buffers outside periods of stress which can be drawn down as losses are incurred. The requirement is based on simple capital conservation rules designed to avoid breaches of minimum capital requirements. Liabilities, fair value disclosure, recurring Financial and Nonfinancial Liabilities, Fair Value Disclosure Related allowance Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Antidilutive securities excluded from computation of earnings per share, amount (in shares) Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Preferred stock, par value (in dollars per share) Preferred Stock, Par or Stated Value Per Share Preferred stock, shares authorized (in shares) Preferred Stock, Shares Authorized Shares issued (in shares) Preferred Stock, Shares Issued Shares outstanding (in shares) Preferred Stock, Shares Outstanding Common stock, par value (in dollars per share) Common Stock, Par or Stated Value Per Share Common stock, shares authorized (in shares) Common Stock, Shares Authorized Common stock, shares issued (in shares) Common Stock, Shares, Issued Common stock, shares outstanding (in shares) Common Stock, Shares, Outstanding Schedule of Maturities of Long-term Debt Schedule of Maturities of Long-term Debt [Table Text Block] Income tax expense Effective income tax rate reconciliation, percent Effective Income Tax Rate Reconciliation, Percent Unrecognized tax benefits, income tax penalties and interest expense Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense Schedule of Earnings Per Share, Basic and Diluted Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Acquisition of F&M Acquisition of F&M [Member] Represents the acquisition of F&M Bancshares, Inc. Goodwill impairment Goodwill, Impaired, Accumulated Impairment Loss Statement of Stockholders' Equity [Abstract] Common Stock Common Stock [Member] Capital Surplus Additional Paid-in Capital [Member] Accumulated Other Comprehensive Income (Loss) AOCI Attributable to Parent [Member] Treasury Stock Treasury Stock [Member] Increase (Decrease) in Stockholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Balance (in shares) Shares, Issued Balance Other comprehensive income (loss) Other Comprehensive Income (Loss), Net of Tax Common stock issued in connection with the exercise of stock options and restricted stock awards (in shares) Stock Issued During Period, Shares, Share-based Compensation, Net of Forfeitures Common stock issued in connection with the exercise of stock options and restricted stock awards Stock Issued During Period, Value, Share-based Compensation, Net of Forfeitures Stock based compensation expense Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition Reclassification of amounts within AOCI to retained earnings due to tax reform (See Note 1) Balance (in shares) Balance Average Recorded Investment Impaired Financing Receivable, Average Recorded Investment Interest Income Recognized Impaired Financing Receivable, Interest Income, Accrual Method Amortized Cost Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Amortized Cost [Abstract] Due in one year or less Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost Due after one year through five years Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost Due after five years through ten years Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost Due after ten years Debt Securities, Available-for-sale, Allocated and Single Maturity Date, Maturity, after 10 Years, Amortized Cost Subtotal Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Amortized Cost Agency mortgage-backed pass through securities Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost Total Debt Securities, Available-for-sale, Amortized Cost Fair Value Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Fair Value [Abstract] Due in one year or less Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value Due after one year through five years Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value Due after five years through ten years Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Fair Value Due after ten years Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value Subtotal Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Fair Value Agency mortgage-backed pass through securities Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value Total