Accounting Policies Significant Accounting Policies (Policies)	3 Months Ended
Mar. 31, 2018
Accounting Policies [Abstract]
Management's Use of Estimates	Management’s Use of Estimates -The preparation of the consolidated financial statements in conformity with accounting principles generally accepted in the United States of America 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 income and expenses during the reporting period. Our most significant estimate is the allowance for loan losses. Other estimates include the evaluation of other-than-temporary impairment of investment securities, goodwill impairment, pension and other postretirement liabilities and an analysis of a need for a valuation allowance for deferred tax assets. Actual results could differ from those estimates. A material estimate that is particularly susceptible to significant change in the near term is the allowance for loan losses. In connection with the determination of the allowance for loan losses, management obtains appraisals for properties. The allowance for loan losses is management’s best estimate of probable loan losses incurred as of the balance sheet date. While management uses available information to recognize losses on loans, future adjustments to the allowance for loan losses may be necessary based on changes in economic conditions.
New Accounting Standards Updates	Recently Adopted and Recently Issued Accounting Standards The following accounting standards have been adopted in the first quarter of 2018: ASU 2016-01 "Recognition and Measurement of Financial Assets and Financial Liabilities" significantly changed the income statement impact of equity investments. For Arrow, the standard became effective for the first quarter of 2018, and requires that equity investments be measured at fair value, with changes in fair value measured in net income. The cumulative effect of the January 1, 2018 adoption was an increase to retained earnings of $331 thousand with a corresponding decrease to Accumulated Other Comprehensive Loss. For periods prior to January 1, 2018, equity securities were classified as available-for-sale and stated at fair value with unrealized gains and losses reported as a separate component of accumulated other comprehensive income, net of tax. ASU 2016-01 also emphasized the existing requirement to use exit prices to measure fair value for disclosure purposes and clarifies that entities not make use of a practicability exception in determining the fair value of loans. Accordingly, we refined the calculation used to determine the disclosed fair value of our loans as part of adopting this standard. See Note 9. Fair Value of Financial Instruments. ASU 2016-15, "Statement of Cash Flows (Topic 230) - Classification of Certain Cash Receipts and Cash Payments" will reduce existing diversity in practice with respect to eight specific cash flow issues. The amendment is effective for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. Early adoption is permitted, including adoption in an interim period. If an entity early adopts the amendments in an interim period, any adjustments should be reflected as of the beginning of the fiscal year that includes the interim period. An entity that elects early adoption must adopt all of the amendments in the same period. The amendments in this update should be applied using a retrospective transition method to each period presented. Arrow adopted this ASU in the first quarter of 2018. ASU 2017-01 "Business Combinations" defines when a set of assets and activities constitutes a business for the purposes of determining whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. Previously, the three elements required to be present in a business are inputs, processes, and outputs. The amendments in this update allow for a business to consist of inputs, processes, and the ability to create output. For Arrow, the standard became effective in the first quarter of 2018. This update had no effect on our accounting for acquisitions and dispositions of businesses. ASU 2017-07 "Compensation-Retirement Benefits" improves the presentation of net periodic pension cost and net periodic post-retirement benefit cost by requiring that an employer disaggregate the service cost component from the other components of net benefit cost. The amendments also provide explicit guidance on how to present the service cost component and the other components of net benefit cost in the income statement and allow only the service cost component of net benefit cost to be eligible for capitalization. For Arrow, the standard became effective in the first quarter of 2018. In accordance with the practical expedient adoption method, for all periods presented Arrow used the amounts disclosed in the retirement plans footnote for the prior period retrospective reclassification of the non-service cost components out of salaries and benefits and into other operating expenses. The adoption of this change in accounting for pension costs did not have a material impact on our financial position or the results of operations. ASU 2017-09 "Compensation-Stock Compensation" provides guidance about which changes to the terms and conditions of a share-based payment award require an entity to apply modification accounting in Topic 718. The guidance highlights the requirements for applying modification accounting and the exception criteria relating to changes in share-based payment terms. For Arrow, the standard became effective in the first quarter of 2018. The adoption of this change in accounting for share-based payment awards did not have a material impact on our financial position or the results of operations in periods subsequent to its adoption. ASU 2014-09 - "Revenue from Contracts with Customers (Topic 606)" clarifies the principles for recognizing revenue and supersedes most current revenue recognition guidance. The ASU is intended to clarify and converge the revenue recognition principles under current generally accepted accounting principles (GAAP) to streamline revenue recognition requirements in addition to expanding required revenue recognition disclosures. In August 2015, the Financial Accounting Standards Board (FASB) issued ASU 2015-14, Deferral of the Effective Date, which provides a one year deferral to the effective date, therefore, ASU 2014-09 is effective for public companies for annual periods, and interim periods within those annual periods, beginning on or after December 15, 2017. As such, the Company adopted ASU 2014-09 as of January 1, 2018 using the modified retrospective approach, and have identified the recognition of revenue related to specific types of fiduciary activities and specific types of revenue from insurance commissions to be in the scope of this guidance. Regarding fiduciary activities, under prior GAAP, revenue was recognized from settling client estates over the time period the work was performed. With the adoption of Topic 606, revenue will be recognized when the performance obligation is completed, which is when the settlement of the client estate is closed. This change will not have a material impact on our consolidated financial statements. Regarding revenue from property and casualty insurance policies in which the client elected to pay premiums in installments, under prior GAAP, revenue was recognized when the client premiums were billed. With the adoption of Topic 606, revenue is recognized when the performance obligation is substantially completed , i.e., when the insurance policy is issued. This change will not have a material impact to our consolidated financial statements. The adoption of Topic 606 related to the previously described fiduciary activity and insurance commission required a cumulative effect adjustment as of January 1, 2018 to decrease retained earnings by $102 thousand. Additional information related to net revenue generated from contracts with customers is as follows: Revenue Recognition - Accounting Standard Codification ("ASC") Topic 606, Revenue from Contracts with Customers ("ASC Topic 606"), establishes principles for reporting information about the nature, amount, timing and uncertainty of revenue and cash flows arising from the entity's contracts to provide goods or services to customers. The core principle requires an entity to recognize revenue to depict the transfer of promised goods or services a customer in an amount that reflects the consideration to which the entity expects to receive in exchange for those goods or services as performance obligations are satisfied. The majority of the Company's revenue-generating transactions are not subject to ASC Topic 606, including revenue generated from financial instruments, such as loans and investment securities which are presented in our consolidated income statements as components of net interest income. The following is a description of principal activities from which the Company generates its revenue from noninterest income sources that are within the scope of ASC Topic 606: Income From Fiduciary Activities: represents revenue derived mainly through the management of client investments which is based on the market value of these assets and the fee schedule contained in the applicable account management agreement. Since the revenue is mainly based on the market value of assets, this amount can be volatile as financial markets increase and decrease based on various economic factors. The terms of the account management agreements generally specify that the performance obligations are completed each quarter. Accordingly, we mainly recognize revenue from fiduciary activities on a quarterly basis. Fees for Other Services to Customers: represents general service fees for monthly deposit account maintenance and account activity plus fees from other deposit-based services. Revenue is recognized when the performance obligation is completed, which is generally on a monthly basis for account maintenance services, or upon the completion of a deposit-related transaction. Payment for such performance obligations are generally received at the time the performance obligations are satisfied. Insurance Commissions: represents commissions and fees paid by insurance carriers for both property and casualty insurance policies, and for services performed for employment benefits clients. Revenue from our property and casualty business is recognized when our performance obligation is satisfied, which is generally the effective date of the bound coverage since there are no significant performance obligations remaining. Revenue from our employment benefit brokerage business is recognized when our benefit servicing performance obligations are satisfied, generally on a monthly basis. The following accounting standards have been issued and become effective for the Company at a future date: ASU 2016-02 "Leases" will require the recognition of operating leases. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842). ASU 2016-02 requires lessees to recognize right of use assets and lease liabilities on the balance sheet for all leases with terms longer than 12 months. For leases with a term of 12 months or less, a lessee is permitted to make an accounting policy election by class of underlying asset not to recognize a right of use asset and lease liability. Additionally, when measuring assets and liabilities arising from a lease, optional payments should be included only if the lessee is reasonably certain to exercise an option to extend the lease, exercise a purchase option or not exercise an option to terminate the lease. In January 2018, the FASB issued ASU 2018-01, Leases (Topic 842): Land Easement Practical Expedient for Transition to Topic 842. ASU 2018-01 was issued to address concerns about the cost and complexity of complying with the transition provisions of ASU 2018-01. Early adoption is permitted in any interim or annual period. For Arrow, the standard becomes effective in the first quarter of 2019. We do not expect that the adoption of this change in accounting for operating leases will have a material impact on our financial position or the results of operations in periods subsequent to its adoption. As of March 31, 2018, we have less than $2.2 million in minimum lease payments for existing operating leases of branch and insurance locations with varying expiration dates from 2018 to 2031. ASU 2016-13 "Financial Instruments - Credit Losses" will change the way we and other financial entities recognize losses on credit instruments, primarily loans and available-for-sale securities. Currently, loan losses are recognized using an "incurred loss" methodology that delays recognition until it is probable a loss has been incurred. Under the ASU, the incurred loss model will be replaced with a methodology that reflects current expected credit losses (CECL) over the life of the instrument and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. Currently, credit losses on available-for-sale securities reduce the carrying value of the instrument and cannot be reversed. Under the ASU, the amount of the credit loss is carried as a valuation allowance and can be reversed. For Arrow, the standard is effective for the first quarter of 2020 and early adoption is allowed in 2019. We plan on adopting the standard in the first quarter of 2020, in order to maximize the accumulation of data needed to calculate the new CECL methodologies. The ASU describes several acceptable methodologies for calculating expected losses on a loan or a pool of loans. Although other methodologies are allowed, we plan on only using the methodologies described in the ASU. Arrow is still evaluating criteria for loan segments and the appropriate methodology for each segment. We expect that the use of the CECL methodology will result in higher levels of the allowance for loan losses than the incurred loss model. The initial adjustment will not be reported in earnings, but as the cumulative effect of a change in accounting principle. We expect that the adoption of the new standard will have a material impact on the level of the allowance for loan losses, but we do not expect that the periodic provision for loan losses to be materially higher than our historic amounts. The adoption of the standard will have an impact on shareholders’ equity, but we expect that we will remain well-capitalized under current regulatory calculations. The standard also requires new disclosures about the loan portfolio. Both the FASB and banking regulators have made estimates of the range of percentage increases in the allowance for loan losses. One FASB board member has commented that the initial range of 30% to 50% for increases in bank allowances is too high, and that the most likely range is in the single digits, although some institutions will experience higher increases. Due to our historical low level of loan losses and our current conservative underwriting standards, we believe that the increase in our loan loss allowance will be in the single digits. The FASB’s Transition Research Group for credit losses still has several outstanding unresolved questions, some of which may have a significant impact on CECL calculations. In 2016 we selected a third-party software product to calculate our CECL methodologies. We also began to accumulate a database of monthly loan data, beginning with June 2015, of the fields needed to make the loss calculations under the new CECL methodologies. During 2018, we plan on refining our loan segment criteria and, for most segments, we will calculate expected losses (for an expanding database) using two or more of the applicable methodologies before determining the most appropriate methodology for each loan segment or sub-segment. ASU 2017-04 "Intangibles-Goodwill and Other" changes the procedures for evaluating impairment of goodwill. Prior to the adoption of this standard, entities were required to perform procedures to determine the fair value of the underlying assets and liabilities following the guidance for determining the fair value of assets and liabilities in a business combination. This additional step to impairment testing has been eliminated. Under this standard, entities should perform goodwill impairment testing by comparing the fair value of a reporting unit to its carrying value. This change should reduce the cost and complexity of evaluating goodwill for impairment. For Arrow, the standard becomes effective in the first quarter of 2019, however, early adoption is permitted. This amendment will not affect our assessment of goodwill impairment since we currently perform the analysis of comparing carrying value to fair value of our reporting units that have goodwill and we have not had to perform a Step 2 Impairment Test to date. ASU 2017-08 "Receivables-Nonrefundable Fees and Other Costs" amends the amortization period for certain purchased callable debt securities held at a premium. This shortens the amortization period for the premium to the earliest call date. Under GAAP, entities generally amortize the premium as an adjustment of yield over the contractual life of the instrument. For Arrow, the standard becomes effective in the first quarter of 2019. We do not expect that the adoption of this change in accounting for certain callable debt securities will have a material impact on our financial position or the results of operations in periods subsequent to its adoption.

Recently Adopted and Recently Issued Accounting Standards

The following accounting standards have been adopted in the first quarter of 2018:

ASU 2016-01 "Recognition and Measurement of Financial Assets and Financial Liabilities" significantly changed the income statement impact of equity investments. For Arrow, the standard became effective for the first quarter of 2018, and requires that equity investments be measured at fair value, with changes in fair value measured in net income. The cumulative effect of the January 1, 2018 adoption was an increase to retained earnings of $331 thousand with a corresponding decrease to Accumulated Other Comprehensive Loss. For periods prior to January 1, 2018, equity securities were classified as available-for-sale and stated at fair value with unrealized gains and losses reported as a separate component of accumulated other comprehensive income, net of tax. ASU 2016-01 also emphasized the existing requirement to use exit prices to measure fair value for disclosure purposes and clarifies that entities not make use of a practicability exception in determining the fair value of loans. Accordingly, we refined the calculation used to determine the disclosed fair value of our loans as part of adopting this standard. See Note 9. Fair Value of Financial Instruments.

ASU 2016-15, "Statement of Cash Flows (Topic 230) - Classification of Certain Cash Receipts and Cash Payments" will reduce existing diversity in practice with respect to eight specific cash flow issues. The amendment is effective for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. Early adoption is permitted, including adoption in an interim period. If an entity early adopts the amendments in an interim period, any adjustments should be reflected as of the beginning of the fiscal year that includes the interim period. An entity that elects early adoption must adopt all of the amendments in the same period. The amendments in this update should be applied using a retrospective transition method to each period presented. Arrow adopted this ASU in the first quarter of 2018.

ASU 2017-01 "Business Combinations" defines when a set of assets and activities constitutes a business for the purposes of determining whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. Previously, the three elements required to be present in a business are inputs, processes, and outputs. The amendments in this update allow for a business to consist of inputs, processes, and the ability to create output. For Arrow, the standard became effective in the first quarter of 2018. This update had no effect on our accounting for acquisitions and dispositions of businesses.

ASU 2017-07 "Compensation-Retirement Benefits" improves the presentation of net periodic pension cost and net periodic post-retirement benefit cost by requiring that an employer disaggregate the service cost component from the other components of net benefit cost. The amendments also provide explicit guidance on how to present the service cost component and the other components of net benefit cost in the income statement and allow only the service cost component of net benefit cost to be eligible for capitalization. For Arrow, the standard became effective in the first quarter of 2018. In accordance with the practical expedient adoption method, for all periods presented Arrow used the amounts disclosed in the retirement plans footnote for the prior period retrospective reclassification of the non-service cost components out of salaries and benefits and into other operating expenses. The adoption of this change in accounting for pension costs did not have a material impact on our financial position or the results of operations.

ASU 2017-09 "Compensation-Stock Compensation" provides guidance about which changes to the terms and conditions of a share-based payment award require an entity to apply modification accounting in Topic 718. The guidance highlights the requirements for applying modification accounting and the exception criteria relating to changes in share-based payment terms. For Arrow, the standard became effective in the first quarter of 2018. The adoption of this change in accounting for share-based payment awards did not have a material impact on our financial position or the results of operations in periods subsequent to its adoption.

ASU 2014-09 - "Revenue from Contracts with Customers (Topic 606)" clarifies the principles for recognizing revenue and supersedes most current revenue recognition guidance. The ASU is intended to clarify and converge the revenue recognition principles under current generally accepted accounting principles (GAAP) to streamline revenue recognition requirements in addition to expanding required revenue recognition disclosures. In August 2015, the Financial Accounting Standards Board (FASB) issued ASU 2015-14, Deferral of the Effective Date, which provides a one year deferral to the effective date, therefore, ASU 2014-09 is effective for public companies for annual periods, and interim periods within those annual periods, beginning on or after December 15, 2017. As such, the Company adopted ASU 2014-09 as of January 1, 2018 using the modified retrospective approach, and have identified the recognition of revenue related to specific types of fiduciary activities and specific types of revenue from insurance commissions to be in the scope of this guidance. Regarding fiduciary activities, under prior GAAP, revenue was recognized from settling client estates over the time period the work was performed. With the adoption of Topic 606, revenue will be recognized when the performance obligation is completed, which is when the settlement of the client estate is closed. This change will not have a material impact on our consolidated financial statements. Regarding revenue from property and casualty insurance policies in which the client elected to pay premiums in installments, under prior GAAP, revenue was recognized when the client premiums were billed. With the adoption of Topic 606, revenue is recognized when the performance obligation is substantially completed , i.e., when the insurance policy is issued. This change will not have a material impact to our consolidated financial statements. The adoption of Topic 606 related to the previously described fiduciary activity and insurance commission required a cumulative effect adjustment as of January 1, 2018 to decrease retained earnings by $102 thousand. Additional information related to net revenue generated from contracts with customers is as follows:

Revenue Recognition - Accounting Standard Codification ("ASC") Topic 606, Revenue from Contracts with Customers ("ASC Topic 606"), establishes principles for reporting information about the nature, amount, timing and uncertainty of revenue and cash flows arising from the entity's contracts to provide goods or services to customers. The core principle requires an entity to recognize revenue to depict the transfer of promised goods or services a customer in an amount that reflects the consideration to which the entity expects to receive in exchange for those goods or services as performance obligations are satisfied.

The majority of the Company's revenue-generating transactions are not subject to ASC Topic 606, including revenue generated from financial instruments, such as loans and investment securities which are presented in our consolidated income statements as components of net interest income. The following is a description of principal activities from which the Company generates its revenue from noninterest income sources that are within the scope of ASC Topic 606:

Income From Fiduciary Activities: represents revenue derived mainly through the management of client investments which is based on the market value of these assets and the fee schedule contained in the applicable account management agreement. Since the revenue is mainly based on the market value of assets, this amount can be volatile as financial markets increase and decrease based on various economic factors. The terms of the account management agreements generally specify that the performance obligations are completed each quarter. Accordingly, we mainly recognize revenue from fiduciary activities on a quarterly basis.

Fees for Other Services to Customers: represents general service fees for monthly deposit account maintenance and account activity plus fees from other deposit-based services. Revenue is recognized when the performance obligation is completed, which is generally on a monthly basis for account maintenance services, or upon the completion of a deposit-related transaction. Payment for such performance obligations are generally received at the time the performance obligations are satisfied.

Insurance Commissions: represents commissions and fees paid by insurance carriers for both property and casualty insurance policies, and for services performed for employment benefits clients. Revenue from our property and casualty business is recognized when our performance obligation is satisfied, which is generally the effective date of the bound coverage since there are no significant performance obligations remaining. Revenue from our employment benefit brokerage business is recognized when our benefit servicing performance obligations are satisfied, generally on a monthly basis.

The following accounting standards have been issued and become effective for the Company at a future date:

ASU 2016-02 "Leases" will require the recognition of operating leases. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842). ASU 2016-02 requires lessees to recognize right of use assets and lease liabilities on the balance sheet for all leases with terms longer than 12 months. For leases with a term of 12 months or less, a lessee is permitted to make an accounting policy election by class of underlying asset not to recognize a right of use asset and lease liability. Additionally, when measuring assets and liabilities arising from a lease, optional payments should be included only if the lessee is reasonably certain to exercise an option to extend the lease, exercise a purchase option or not exercise an option to terminate the lease. In January 2018, the FASB issued ASU 2018-01, Leases (Topic 842): Land Easement Practical Expedient for Transition to Topic 842. ASU 2018-01 was issued to address concerns about the cost and complexity of complying with the transition provisions of ASU 2018-01. Early adoption is permitted in any interim or annual period. For Arrow, the standard becomes effective in the first quarter of 2019. We do not expect that the adoption of this change in accounting for operating leases will have a material impact on our financial position or the results of operations in periods subsequent to its adoption. As of March 31, 2018, we have less than $2.2 million in minimum lease payments for existing operating leases of branch and insurance locations with varying expiration dates from 2018 to 2031.

ASU 2016-13 "Financial Instruments - Credit Losses" will change the way we and other financial entities recognize losses on credit instruments, primarily loans and available-for-sale securities. Currently, loan losses are recognized using an "incurred loss" methodology that delays recognition until it is probable a loss has been incurred. Under the ASU, the incurred loss model will be replaced with a methodology that reflects current expected credit losses (CECL) over the life of the instrument and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. Currently, credit losses on available-for-sale securities reduce the carrying value of the instrument and cannot be reversed. Under the ASU, the amount of the credit loss is carried as a valuation allowance and can be reversed. For Arrow, the standard is effective for the first quarter of 2020 and early adoption is allowed in 2019. We plan on adopting the standard in the first quarter of 2020, in order to maximize the accumulation of data needed to calculate the new CECL methodologies. The ASU describes several acceptable methodologies for calculating expected losses on a loan or a pool of loans. Although other methodologies are allowed, we plan on only using the methodologies described in the ASU. Arrow is still evaluating criteria for loan segments and the appropriate methodology for each segment. We expect that the use of the CECL methodology will result in higher levels of the allowance for loan losses than the incurred loss model. The initial adjustment will not be reported in earnings, but as the cumulative effect of a change in accounting principle. We expect that the adoption of the new standard will have a material impact on the level of the allowance for loan losses, but we do not expect that the periodic provision for loan losses to be materially higher than our historic amounts. The adoption of the standard will have an impact on shareholders’ equity, but we expect that we will remain well-capitalized under current regulatory calculations. The standard also requires new disclosures about the loan portfolio. Both the FASB and banking regulators have made estimates of the range of percentage increases in the allowance for loan losses. One FASB board member has commented that the initial range of 30% to 50% for increases in bank allowances is too high, and that the most likely range is in the single digits, although some institutions will experience higher increases. Due to our historical low level of loan losses and our current conservative underwriting standards, we believe that the increase in our loan loss allowance will be in the single digits. The FASB’s Transition Research Group for credit losses still has several outstanding unresolved questions, some of which may have a significant impact on CECL calculations. In 2016 we selected a third-party software product to calculate our CECL methodologies. We also began to accumulate a database of monthly loan data, beginning with June 2015, of the fields needed to make the loss calculations under the new CECL methodologies. During 2018, we plan on refining our loan segment criteria and, for most segments, we will calculate expected losses (for an expanding database) using two or more of the applicable methodologies before determining the most appropriate methodology for each loan segment or sub-segment.

ASU 2017-04 "Intangibles-Goodwill and Other" changes the procedures for evaluating impairment of goodwill. Prior to the adoption of this standard, entities were required to perform procedures to determine the fair value of the underlying assets and liabilities following the guidance for determining the fair value of assets and liabilities in a business combination. This additional step to impairment testing has been eliminated. Under this standard, entities should perform goodwill impairment testing by comparing the fair value of a reporting unit to its carrying value. This change should reduce the cost and complexity of evaluating goodwill for impairment. For Arrow, the standard becomes effective in the first quarter of 2019, however, early adoption is permitted. This amendment will not affect our assessment of goodwill impairment since we currently perform the analysis of comparing carrying value to fair value of our reporting units that have goodwill and we have not had to perform a Step 2 Impairment Test to date.

ASU 2017-08 "Receivables-Nonrefundable Fees and Other Costs" amends the amortization period for certain purchased callable debt securities held at a premium. This shortens the amortization period for the premium to the earliest call date. Under GAAP, entities generally amortize the premium as an adjustment of yield over the contractual life of the instrument. For Arrow, the standard becomes effective in the first quarter of 2019. We do not expect that the adoption of this change in accounting for certain callable debt securities will have a material impact on our financial position or the results of operations in periods subsequent to its adoption.