Accounting Policies Significant Accounting Policies (Policies)	3 Months Ended
Mar. 31, 2019
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.
Recently Adopted and Recently Issued Accounting Standards	Recently Adopted and Recently Issued Accounting Standards The following accounting standards have been adopted in the first three months of 2019: In March 2017, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2017-08 "Receivables-Nonrefundable Fees and Other Costs" which 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 United States generally accepted accounting principles (GAAP), entities generally amortize the premium as an adjustment of yield over the contractual life of the instrument. For Arrow, the standard was effective in the first quarter of 2019 and did not have a material impact on its financial position or the results of operations in the current quarter or in future periods. In February 2016, the FASB issued ASU 2016-02, "Leases" (Topic 842) which requires lessees to recognize right-of-use assets and lease liabilities on the balance sheet for all leases with terms longer than 12 months. In January 2018, the FASB issued ASU 2018-01, Leases (Topic 842): "Land Easement Practical Expedient for Transition to Topic 842". In July 2018, the FASB issued ASU 2018-10 "Codification Improvements to Topic 842, Leases" which provided clarification on certain components of the original guidance, including that the rate implicit in the lease cannot be less than zero. Also in July 2018, the FASB issued ASU 2018-11 "Targeted Improvements" to Leases (Topic 842) which amends the original guidance to allow for the adoption of this standard to be applied retrospectively at the beginning of the period of adoption, which was January 1, 2019 for Arrow, without revising prior comparative periods. The Company adopted this standard as of January 1, 2019 using the effective date method, also known as the modified retrospective method, with the cumulative-effect adjustment recorded at the beginning of the period of adoption. As a result of this adoption, the Company's assets increased $7.9 million and the Company's liabilities increased $8.0 million with no adjustment required to retained earnings and no material impact to the Consolidated Statements of Income. Practical Expedients Elected At Adoption: The package of practical expedients were elected that did not require the Company to reassess whether an existing contract contains a lease, to reassess existing leases between operating leases and finance leases and to not reassess initial direct costs for any existing leases. These practical expedients were applied together. In addition, the Company also elected a practical expedient, which was required to be applied consistently to all of its leases, to use hindsight in determining the lease term when considering lessee options to extend or terminate the lease and in assessing impairment in the right-of-use asset. Accounting Policy Elections: The Company also made two accounting policy elections related to the adoption of this standard. The first is a determination not to separate lease and non-lease components and account for the resulting combined component as a single lease component. The second election is to account for short-term leases, those leases with a "lease term" of twelve months or less, like an operating lease under current GAAP. Determination of the Discount Rate to Calculate the Lease Liability: Since the Company was unable to determine the rate implicit in its leases, the secured borrowing rate from the Federal Home Loan Bank of New York as of the January 1, 2019 adoption date was utilized for existing leases for the effective lease term beginning with the effective date of each existing lease. The expected expiration date of each lease was determined on a lease-by-lease basis based on the availability of renewal options in the lease contracts, the amount of leasehold improvements at each location, total branch deposits at each location in addition to the feasibility of growth potential at each location. A similar process is followed to determine the expected lease expiration date for all leases executed subsequent to the January 1, 2019 adoption date. The following accounting standards have been issued and will become effective for the Company at a future date: In June 2016, the FASB issued ASU 2016-13 "Financial Instruments - Credit Losses" (Topic 326) which will change the way financial entities measure expected credit losses for financial assets, primarily loans. Under this ASU, the "incurred loss" model will be replaced with an "expected loss" model which will recognize losses over the life of the instrument and requires consideration of a broader range of reasonable and supportable information. Currently, credit losses on available-for-sale securities reduce the carrying value of the instrument and cannot be reversed. Under this ASU, the amount of the credit loss is carried as a valuation allowance and can be reversed. The standard also requires expanded credit quality disclosures. For Arrow, the standard is effective for the first quarter of 2020 and early adoption is allowed in 2019. The Company plans on adopting the standard in the first quarter of 2020, in order to maximize the accumulation of data needed to calculate the new current expected credit loss (CECL) methodologies. The ASU describes several acceptable methodologies for calculating expected losses on a loan or a pool of loans and requires additional disclosures. The initial adjustment will not be reported in earnings, but as the cumulative effect of a change in accounting principle. 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. The Company has continued its implementation efforts with the development and testing of various methods within its core model, and has tentatively identified the discounted cash flow method for determining losses for the commercial loan portfolios and the residential real estate portfolios, and the vintage method for the consumer indirect loan portfolio. Based on further testing, these methods may change prior to adoption. As a result of analyses performed, including the availability of future economic data, the Company plans to utilize a two-year reasonable and supportable forecast period and is in the process of identifying which economic data best correlates with expected loan losses. The Company continues to monitor new regulatory guidance and is updating relevant internal controls and processes. The adoption of this standard will likely have the effect of increasing the allowance for loan and lease losses and reducing shareholders' equity, the extent of which will depend upon the nature and characteristics of the Company's loan portfolio and economic conditions and forecasts at the adoption date. The Company expects to remain a well-capitalized financial institution under current regulatory calculations. In August 2018, the FASB issued ASU 2018-13 "Disclosure Framework-Changes to the Disclosure Requirements for Fair Value Measurement" which as part of its disclosure framework, the FASB has eliminated, amended and added disclosure requirements for fair value measurements. The following disclosure requirements were eliminated: Amount of and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy; the policy of the timing of transfers between levels of the fair value hierarchy; the valuation processes for Level 3 fair value measurements. For public companies such as Arrow, the following new disclosures will be required: Changes in unrealized gains and losses for the period included in other comprehensive income (OCI); the range and weighted average of significant unobservable inputs used; alternatively, a company may choose to disclose other quantitative information if it determines that it is a more reasonable and rational method that reflects the distribution of unobservable inputs used. For Arrow, the standard becomes effective in the first quarter of 2020. The Company does not expect that the adoption of this change in fair value disclosure will have a material impact on its financial position or the results of operations in periods subsequent to its adoption. In August 2018, the FASB issued ASU 2018-14 "Disclosure Framework-Changes to the Disclosure Requirements for Defined Benefit Plans" which applies to all companies that provide defined benefit pension or other postretirement benefit plans for their employees. Certain disclosure requirements have been eliminated such as reporting the amounts in accumulated other comprehensive income expected to be recognized as components of net periodic benefit cost over the next year, and reporting the effects of a one-percentage-point change in the assumed healthcare cost trend rate on the aggregate of the service cost and interest cost components of net periodic benefit cost and on the benefit obligation for postretirement healthcare benefits. New required disclosures for reporting the weighted-average interest rate used to credit cash balance and similar plans that have a promised interest credit, the reasons for significant gains and losses affecting benefit obligations and other requirements for reporting aggregate information related to pension plans. For Arrow, the standard becomes effective at December 31, 2020. The Company does not expect that the adoption of this change affecting defined benefit plan disclosures will have a material impact on its financial position or the results of operations. In August 2018, the FASB issued ASU 2018-15 "Customer's Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract" which will require companies to defer potentially significant, specified implementation costs incurred in a cloud computing arrangement that are often expensed under current US GAAP. For Arrow, the standard becomes effective at January 1, 2020. The Company is in the process of assessing the impact of this new accounting standard on its financial position and 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 three months of 2019:

In March 2017, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2017-08 "Receivables-Nonrefundable Fees and Other Costs" which 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 United States generally accepted accounting principles (GAAP), entities generally amortize the premium as an adjustment of yield over the contractual life of the instrument. For Arrow, the standard was effective in the first quarter of 2019 and did not have a material impact on its financial position or the results of operations in the current quarter or in future periods.

In February 2016, the FASB issued ASU 2016-02, "Leases" (Topic 842) which requires lessees to recognize right-of-use assets and lease liabilities on the balance sheet for all leases with terms longer than 12 months. In January 2018, the FASB issued ASU 2018-01, Leases (Topic 842): "Land Easement Practical Expedient for Transition to Topic 842". In July 2018, the FASB issued ASU 2018-10 "Codification Improvements to Topic 842, Leases" which provided clarification on certain components of the original guidance, including that the rate implicit in the lease cannot be less than zero. Also in July 2018, the FASB issued ASU 2018-11 "Targeted Improvements" to Leases (Topic 842) which amends the original guidance to allow for the adoption of this standard to be applied retrospectively at the beginning of the period of adoption, which was January 1, 2019 for Arrow, without revising prior comparative periods.

The Company adopted this standard as of January 1, 2019 using the effective date method, also known as the modified retrospective method, with the cumulative-effect adjustment recorded at the beginning of the period of adoption. As a result of this adoption, the Company's assets increased $7.9 million and the Company's liabilities increased $8.0 million with no adjustment required to retained earnings and no material impact to the Consolidated Statements of Income.

Practical Expedients Elected At Adoption: The package of practical expedients were elected that did not require the Company to reassess whether an existing contract contains a lease, to reassess existing leases between operating leases and finance leases and to not reassess initial direct costs for any existing leases. These practical expedients were applied together. In addition, the Company also elected a practical expedient, which was required to be applied consistently to all of its leases, to use hindsight in determining the lease term when considering lessee options to extend or terminate the lease and in assessing impairment in the right-of-use asset.

Accounting Policy Elections: The Company also made two accounting policy elections related to the adoption of this standard. The first is a determination not to separate lease and non-lease components and account for the resulting combined component as a single lease component. The second election is to account for short-term leases, those leases with a "lease term" of twelve months or less, like an operating lease under current GAAP.

Determination of the Discount Rate to Calculate the Lease Liability: Since the Company was unable to determine the rate implicit in its leases, the secured borrowing rate from the Federal Home Loan Bank of New York as of the January 1, 2019 adoption date was utilized for existing leases for the effective lease term beginning with the effective date of each existing lease. The expected expiration date of each lease was determined on a lease-by-lease basis based on the availability of renewal options in the lease contracts, the amount of leasehold improvements at each location, total branch deposits at each location in addition to the feasibility of growth potential at each location. A similar process is followed to determine the expected lease expiration date for all leases executed subsequent to the January 1, 2019 adoption date.

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

In June 2016, the FASB issued ASU 2016-13 "Financial Instruments - Credit Losses" (Topic 326) which will change the way financial entities measure expected credit losses for financial assets, primarily loans. Under this ASU, the "incurred loss" model will be replaced with an "expected loss" model which will recognize losses over the life of the instrument and requires consideration of a broader range of reasonable and supportable information. Currently, credit losses on available-for-sale securities reduce the carrying value of the instrument and cannot be reversed. Under this ASU, the amount of the credit loss is carried as a valuation allowance and can be reversed. The standard also requires expanded credit quality disclosures. For Arrow, the standard is effective for the first quarter of 2020 and early adoption is allowed in 2019. The Company plans on adopting the standard in the first quarter of 2020, in order to maximize the accumulation of data needed to calculate the new current expected credit loss (CECL) methodologies. The ASU describes several acceptable methodologies for calculating expected losses on a loan or a pool of loans and requires additional disclosures. The initial adjustment will not be reported in earnings, but as the cumulative effect of a change in accounting principle. 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. The Company has continued its implementation efforts with the development and testing of various methods within its core model, and has tentatively identified the discounted cash flow method for determining losses for the commercial loan portfolios and the residential real estate portfolios, and the vintage method for the consumer indirect loan portfolio. Based on further testing, these methods may change prior to adoption. As a result of analyses performed, including the availability of future economic data, the Company plans to utilize a two-year reasonable and supportable forecast period and is in the process of identifying which economic data best correlates with expected loan losses. The Company continues to monitor new regulatory guidance and is updating relevant internal controls and processes. The adoption of this standard will likely have the effect of increasing the allowance for loan and lease losses and reducing shareholders' equity, the extent of which will depend upon the nature and characteristics of the Company's loan portfolio and economic conditions and forecasts at the adoption date. The Company expects to remain a well-capitalized financial institution under current regulatory calculations.

In August 2018, the FASB issued ASU 2018-13 "Disclosure Framework-Changes to the Disclosure Requirements for Fair Value Measurement" which as part of its disclosure framework, the FASB has eliminated, amended and added disclosure requirements for fair value measurements. The following disclosure requirements were eliminated: Amount of and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy; the policy of the timing of transfers between levels of the fair value hierarchy; the valuation processes for Level 3 fair value measurements. For public companies such as Arrow, the following new disclosures will be required: Changes in unrealized gains and losses for the period included in other comprehensive income (OCI); the range and weighted average of significant unobservable inputs used; alternatively, a company may choose to disclose other quantitative information if it determines that it is a more reasonable and rational method that reflects the distribution of unobservable inputs used. For Arrow, the standard becomes effective in the first quarter of 2020. The Company does not expect that the adoption of this change in fair value disclosure will have a material impact on its financial position or the results of operations in periods subsequent to its adoption.

In August 2018, the FASB issued ASU 2018-14 "Disclosure Framework-Changes to the Disclosure Requirements for Defined Benefit Plans" which applies to all companies that provide defined benefit pension or other postretirement benefit plans for their employees. Certain disclosure requirements have been eliminated such as reporting the amounts in accumulated other comprehensive income expected to be recognized as components of net periodic benefit cost over the next year, and reporting the effects of a one-percentage-point change in the assumed healthcare cost trend rate on the aggregate of the service cost and interest cost components of net periodic benefit cost and on the benefit obligation for postretirement healthcare benefits. New required disclosures for reporting the weighted-average interest rate used to credit cash balance and similar plans that have a promised interest credit, the reasons for significant gains and losses affecting benefit obligations and other requirements for reporting aggregate information related to pension plans. For Arrow, the standard becomes effective at December 31, 2020. The Company does not expect that the adoption of this change affecting defined benefit plan disclosures will have a material impact on its financial position or the results of operations.

In August 2018, the FASB issued ASU 2018-15 "Customer's Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract" which will require companies to defer potentially significant, specified implementation costs incurred in a cloud computing arrangement that are often expensed under current US GAAP. For Arrow, the standard becomes effective at January 1, 2020. The Company is in the process of assessing the impact of this new accounting standard on its financial position and the results of operations in periods subsequent to its adoption.