Basis of Presentation (Policies)	9 Months Ended
Sep. 30, 2017
Investments, Debt and Equity Securities [Abstract]
Basis of Presentation	The accompanying consolidated financial statements are unaudited and include the accounts of Alexander’s and its consolidated subsidiaries. All intercompany amounts have been eliminated. In our opinion, all adjustments (which include only normal recurring adjustments) necessary to present fairly the financial position, results of operations and changes in cash flows have been made. Certain information and footnote disclosures normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) have been condensed or omitted. These condensed consolidated financial statements have been prepared in accordance with the instructions to Form 10-Q of the Securities and Exchange Commission (the “SEC”) and should be read in conjunction with the consolidated financial statements and notes thereto included in our Annual Report on Form 10-K for the year ended December 31, 2016, as filed with the SEC. We have made 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 revenues and expenses during the reporting periods. Actual results could differ from those estimates. The results of operations for the three and nine months ended September 30, 2017 are not necessarily indicative of the operating results for the full year. We operate in one reportable segment.
Revenue Recognition	In May 2014, the Financial Accounting Standards Board (“FASB”) issued an update (“ASU 2014-09”) establishing Accounting Standards Codification (“ASC”) Topic 606, Revenue from Contracts with Customers (“ASC 606”). ASU 2014-09, as amended by subsequent ASUs on the topic, establishes a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers and supersedes most of the existing revenue recognition guidance. This standard, which is effective for interim and annual reporting periods in fiscal years that begin after December 15, 2017, requires an entity to recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services and also requires certain additional disclosures. We will adopt this standard effective January 1, 2018, with the exception of the components of revenue from leases, which has been deferred until the adoption of the update ASU 2016-02 to ASC Topic 842, Leases, on January 1, 2019. We will utilize the modified retrospective method when adopting ASU 2014-09, which requires a cumulative-effect adjustment to retained earnings as of the beginning of the fiscal year of adoption. We have analyzed our revenue streams and identified the areas that we expect to be impacted by the adoption of this standard. We expect that this standard will have an impact on the classification of reimbursements of real estate taxes and insurance expenses and certain non-lease components of revenue (e.g., reimbursements of common area maintenance expenses) from leases on our consolidated statements of income, with no impact on “total revenues” for new leases executed on or after January 1, 2019. We are in the process of completing the evaluation of the overall impact of this standard on our consolidated financial statements, including required informational disclosures for our revenue streams beginning with the first reporting period after adoption.
Fair value of financial instruments	In January 2016, the FASB issued an update (“ASU 2016-01”) Recognition and Measurement of Financial Assets and Financial Liabilities to ASC Topic 825, Financial Instruments. ASU 2016-01 amends certain aspects of recognition, measurement, presentation and disclosure of financial instruments. ASU 2016-01 is effective for interim and annual reporting periods in fiscal years beginning after December 15, 2017. While the adoption of this standard requires us to continue to measure “marketable securities” at fair value at each reporting date, the changes in fair value will be recognized in current period earnings as opposed to “other comprehensive income (loss).”
Lessee Leases	In February 2016, the FASB issued an update ASU 2016-02 establishing ASC Topic 842, Leases, which sets out the principles for the recognition, measurement, presentation and disclosure of leases for both lessees and lessors. ASU 2016-02 requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase. Lessees are required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases. Lessees will recognize expense based on the effective interest method for finance leases or on a straight-line basis for operating leases. We are currently evaluating the overall impact of the adoption of ASU 2016-02 on our consolidated financial statements and believe that the standard will more significantly impact the accounting for leases in which we are a lessee. We will be required to record a right-of-use asset and lease liability for our Flushing property ground lease, equal to the present value of the remaining minimum lease payments upon adoption of this standard. We also expect that this standard will require us to allocate total consideration from leases between lease and non-lease components based on the estimated stand-alone selling prices of the components. The lease components (e.g., base rent) will continue to be recognized on a straight-line basis over the term of the lease and certain non-lease components (e.g., reimbursements of common area maintenance expenses) will be accounted for under the new revenue recognition guidance of ASU 2014-09. As a result, we expect that this standard will have an impact on the classification of reimbursements of real estate taxes, insurance expenses and common area maintenance expenses on our consolidated statements of income, with no impact on “total revenues” for new leases executed on or after January 1, 2019. ASU 2016-02 is effective for reporting periods beginning after December 15, 2018, with early adoption permitted. We will adopt this standard as of January 1, 2019 under the modified retrospective approach and will elect to use the practical expedients provided by this standard.
Share-based Compensation, Option and Incentive Plans	In March 2016, the FASB issued an update (“ASU 2016-09”) Improvements to Employee Share-Based Payment Accounting to ASC Topic 718, Compensation – Stock Compensation (“ASC 718”). ASU 2016-09 amends several aspects of the accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. ASU 2016-09 was effective for interim and annual reporting periods in fiscal years beginning after December 15, 2016. The adoption of this update as of January 1, 2017, did not have any impact on our consolidated financial statements.
Cash and Cash Equivalents	In August 2016, the FASB issued an update (“ASU 2016-15”) Classification of Certain Cash Receipts and Cash Payments to ASC Topic 230, Statement of Cash Flows. ASU 2016-15 clarifies guidance on the classification of certain cash receipts and payments in the statement of cash flows to reduce diversity in practice with respect to (i) debt prepayment or debt extinguishment costs, (ii) settlement of zero-coupon debt instruments or other debt instruments with coupon interest rates that are insignificant in relation to the effective interest rate of the borrowing, (iii) contingent consideration payments made after a business combination, (iv) proceeds from the settlement of insurance claims, (v) proceeds from the settlement of corporate-owned life insurance policies, including bank-owned life insurance policies, (vi) distributions received from equity method investees, (vii) beneficial interests in securitization transactions, and (viii) separately identifiable cash flows and application of the predominance principle. ASU 2016-15 is effective for interim and annual reporting periods in fiscal years beginning after December 15, 2017, with early adoption permitted. We elected to early adopt ASU 2016-15 effective January 1, 2017. The adoption of this update did not have a material impact on our consolidated financial statements.
Restricted Cash	In November 2016, the FASB issued an update (“ASU 2016-18”) Restricted Cash to ASC Topic 230, Statement of Cash Flows. ASU 2016-18 requires that a statement of cash flows explain the change during the period in the total of cash, cash equivalents and amounts generally described as restricted cash or restricted cash equivalents. Restricted cash and restricted cash equivalents will be included with cash and cash equivalents when reconciling the beginning of period and end of period balances on the statement of cash flows upon adoption of this standard. ASU 2016-18 is effective for interim and annual reporting periods in fiscal years beginning after December 15, 2017, with early adoption permitted. We elected to early adopt ASU 2016-18 effective January 1, 2017, with retrospective application to our consolidated statements of cash flows. Accordingly, the consolidated statements of cash flows present a reconciliation of the changes in cash and cash equivalents and restricted cash. Restricted cash primarily consists of cash held in a non-interest bearing escrow account in connection with our Rego Park I 100% cash collateralized mortgage, as well as security deposits and other cash escrowed under loan agreements for debt service, real estate taxes, property insurance and capital improvements.
Recently Issued Accounting Literature	In February 2017, the FASB issued an update (“ASU 2017-05”) Clarifying the Scope of Asset Derecognition Guidance and Accounting for Partial Sales of Nonfinancial Assets to ASC Subtopic 610-20, Other Income – Gains and Losses from the Derecognition of Nonfinancial Assets. ASU 2017-05 clarifies the scope of recently established guidance on nonfinancial asset derecognition as well as the accounting for partial sales of nonfinancial assets. This update conforms the derecognition guidance on nonfinancial assets with the model for transactions in ASC 606. ASU 2017-05 is effective for interim and annual reporting periods in fiscal years beginning after December 15, 2017. The adoption of this standard is not expected to have an impact on our consolidated financial statements.
Hedging Activities	In August 2017, the FASB issued an update (“ASU 2017-12”) Targeted Improvements to Accounting for Hedging Activities to ASC Topic 815, Derivatives and Hedging (“ASC 815”). ASU 2017-12 amends the hedge accounting recognition and presentation requirements in ASC 815. The update is intended to more closely align hedge accounting with companies’ risk management strategies, simplify the application of hedge accounting and increase transparency as to the scope and results of hedge programs. ASU 2017-12 is effective for interim and annual reporting periods in fiscal years beginning after December 15, 2018, with early adoption permitted. The adoption of this standard is not expected to have a material impact on our consolidated financial statements.
Marketable Securities	Available-for-sale securities are presented at fair value and unrealized gains and losses resulting from the mark-to-market of these securities are included in “other comprehensive (loss) income.”
Fair Value Measurement	ASC 820, Fair Value Measurements and Disclosures defines fair value and establishes a framework for measuring fair value. ASC 820 establishes a fair value hierarchy that prioritizes observable and unobservable inputs used to measure fair value into three levels: Level 1 – quoted prices (unadjusted) in active markets that are accessible at the measurement date for assets or liabilities; Level 2 – observable prices that are based on inputs not quoted in active markets, but corroborated by market data; and Level 3 – unobservable inputs that are used when little or no market data is available. The fair value hierarchy gives the highest priority to Level 1 inputs and the lowest priority to Level 3 inputs. In determining fair value, we utilize valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs to the extent possible, as well as consider counterparty credit risk in our assessment of fair value.

Basis of Presentation (Policies)

9 Months Ended

Sep. 30, 2017

Investments, Debt and Equity Securities [Abstract]

The accompanying consolidated financial statements are unaudited and include the accounts of Alexander’s and its consolidated subsidiaries. All intercompany amounts have been eliminated. In our opinion, all adjustments (which include only normal recurring adjustments) necessary to present fairly the financial position, results of operations and changes in cash flows have been made. Certain information and footnote disclosures normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) have been condensed or omitted. These condensed consolidated financial statements have been prepared in accordance with the instructions to Form 10-Q of the Securities and Exchange Commission (the “SEC”) and should be read in conjunction with the consolidated financial statements and notes thereto included in our Annual Report on Form 10-K for the year ended December 31, 2016, as filed with the SEC.

We have made 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 revenues and expenses during the reporting periods. Actual results could differ from those estimates. The results of operations for the three and nine months ended September 30, 2017 are not necessarily indicative of the operating results for the full year.

We operate in one reportable segment.

Revenue Recognition

In May 2014, the Financial Accounting Standards Board (“FASB”) issued an update (“ASU 2014-09”) establishing Accounting Standards Codification (“ASC”) Topic 606, Revenue from Contracts with Customers (“ASC 606”). ASU 2014-09, as amended by subsequent ASUs on the topic, establishes a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers and supersedes most of the existing revenue recognition guidance. This standard, which is effective for interim and annual reporting periods in fiscal years that begin after December 15, 2017, requires an entity to recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services and also requires certain additional disclosures. We will adopt this standard effective January 1, 2018, with the exception of the components of revenue from leases, which has been deferred until the adoption of the update ASU 2016-02 to ASC Topic 842, Leases, on January 1, 2019. We will utilize the modified retrospective method when adopting ASU 2014-09, which requires a cumulative-effect adjustment to retained earnings as of the beginning of the fiscal year of adoption. We have analyzed our revenue streams and identified the areas that we expect to be impacted by the adoption of this standard. We expect that this standard will have an impact on the classification of reimbursements of real estate taxes and insurance expenses and certain non-lease components of revenue (e.g., reimbursements of common area maintenance expenses) from leases on our consolidated statements of income, with no impact on “total revenues” for new leases executed on or after January 1, 2019. We are in the process of completing the evaluation of the overall impact of this standard on our consolidated financial statements, including required informational disclosures for our revenue streams beginning with the first reporting period after adoption.

Fair value of financial instruments

In January 2016, the FASB issued an update (“ASU 2016-01”) Recognition and Measurement of Financial Assets and Financial Liabilities to ASC Topic 825, Financial Instruments. ASU 2016-01 amends certain aspects of recognition, measurement, presentation and disclosure of financial instruments. ASU 2016-01 is effective for interim and annual reporting periods in fiscal years beginning after December 15, 2017. While the adoption of this standard requires us to continue to measure “marketable securities” at fair value at each reporting date, the changes in fair value will be recognized in current period earnings as opposed to “other comprehensive income (loss).”

Lessee Leases

In February 2016, the FASB issued an update ASU 2016-02 establishing ASC Topic 842, Leases, which sets out the principles for the recognition, measurement, presentation and disclosure of leases for both lessees and lessors. ASU 2016-02 requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase. Lessees are required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases. Lessees will recognize expense based on the effective interest method for finance leases or on a straight-line basis for operating leases. We are currently evaluating the overall impact of the adoption of ASU 2016-02 on our consolidated financial statements and believe that the standard will more significantly impact the accounting for leases in which we are a lessee. We will be required to record a right-of-use asset and lease liability for our Flushing property ground lease, equal to the present value of the remaining minimum lease payments upon adoption of this standard. We also expect that this standard will require us to allocate total consideration from leases between lease and non-lease components based on the estimated stand-alone selling prices of the components. The lease components (e.g., base rent) will continue to be recognized on a straight-line basis over the term of the lease and certain non-lease components (e.g., reimbursements of common area maintenance expenses) will be accounted for under the new revenue recognition guidance of ASU 2014-09. As a result, we expect that this standard will have an impact on the classification of reimbursements of real estate taxes, insurance expenses and common area maintenance expenses on our consolidated statements of income, with no impact on “total revenues” for new leases executed on or after January 1, 2019. ASU 2016-02 is effective for reporting periods beginning after December 15, 2018, with early adoption permitted. We will adopt this standard as of January 1, 2019 under the modified retrospective approach and will elect to use the practical expedients provided by this standard.

Share-based Compensation, Option and Incentive Plans

In March 2016, the FASB issued an update (“ASU 2016-09”) Improvements to Employee Share-Based Payment Accounting to ASC Topic 718, Compensation – Stock Compensation (“ASC 718”). ASU 2016-09 amends several aspects of the accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. ASU 2016-09 was effective for interim and annual reporting periods in fiscal years beginning after December 15, 2016. The adoption of this update as of January 1, 2017, did not have any impact on our consolidated financial statements.

Cash and Cash Equivalents

In August 2016, the FASB issued an update (“ASU 2016-15”) Classification of Certain Cash Receipts and Cash Payments to ASC Topic 230, Statement of Cash Flows. ASU 2016-15 clarifies guidance on the classification of certain cash receipts and payments in the statement of cash flows to reduce diversity in practice with respect to (i) debt prepayment or debt extinguishment costs, (ii) settlement of zero-coupon debt instruments or other debt instruments with coupon interest rates that are insignificant in relation to the effective interest rate of the borrowing, (iii) contingent consideration payments made after a business combination, (iv) proceeds from the settlement of insurance claims, (v) proceeds from the settlement of corporate-owned life insurance policies, including bank-owned life insurance policies, (vi) distributions received from equity method investees, (vii) beneficial interests in securitization transactions, and (viii) separately identifiable cash flows and application of the predominance principle. ASU 2016-15 is effective for interim and annual reporting periods in fiscal years beginning after December 15, 2017, with early adoption permitted. We elected to early adopt ASU 2016-15 effective January 1, 2017. The adoption of this update did not have a material impact on our consolidated financial statements.

Restricted Cash

In November 2016, the FASB issued an update (“ASU 2016-18”) Restricted Cash to ASC Topic 230, Statement of Cash Flows. ASU 2016-18 requires that a statement of cash flows explain the change during the period in the total of cash, cash equivalents and amounts generally described as restricted cash or restricted cash equivalents. Restricted cash and restricted cash equivalents will be included with cash and cash equivalents when reconciling the beginning of period and end of period balances on the statement of cash flows upon adoption of this standard. ASU 2016-18 is effective for interim and annual reporting periods in fiscal years beginning after December 15, 2017, with early adoption permitted. We elected to early adopt ASU 2016-18 effective January 1, 2017, with retrospective application to our consolidated statements of cash flows. Accordingly, the consolidated statements of cash flows present a reconciliation of the changes in cash and cash equivalents and restricted cash. Restricted cash primarily consists of cash held in a non-interest bearing escrow account in connection with our Rego Park I 100% cash collateralized mortgage, as well as security deposits and other cash escrowed under loan agreements for debt service, real estate taxes, property insurance and capital improvements.

Recently Issued Accounting Literature

In February 2017, the FASB issued an update (“ASU 2017-05”) Clarifying the Scope of Asset Derecognition Guidance and Accounting for Partial Sales of Nonfinancial Assets to ASC Subtopic 610-20, Other Income – Gains and Losses from the Derecognition of Nonfinancial Assets. ASU 2017-05 clarifies the scope of recently established guidance on nonfinancial asset derecognition as well as the accounting for partial sales of nonfinancial assets. This update conforms the derecognition guidance on nonfinancial assets with the model for transactions in ASC 606. ASU 2017-05 is effective for interim and annual reporting periods in fiscal years beginning after December 15, 2017. The adoption of this standard is not expected to have an impact on our consolidated financial statements.

Hedging Activities

In August 2017, the FASB issued an update (“ASU 2017-12”) Targeted Improvements to Accounting for Hedging Activities to ASC Topic 815, Derivatives and Hedging (“ASC 815”). ASU 2017-12 amends the hedge accounting recognition and presentation requirements in ASC 815. The update is intended to more closely align hedge accounting with companies’ risk management strategies, simplify the application of hedge accounting and increase transparency as to the scope and results of hedge programs. ASU 2017-12 is effective for interim and annual reporting periods in fiscal years beginning after December 15, 2018, with early adoption permitted. The adoption of this standard is not expected to have a material impact on our consolidated financial statements.

Marketable Securities

Available-for-sale securities are presented at fair value and unrealized gains and losses resulting from the mark-to-market of these securities are included in “other comprehensive (loss) income.”

Fair Value Measurement

ASC 820, Fair Value Measurements and Disclosures defines fair value and establishes a framework for measuring fair value. ASC 820 establishes a fair value hierarchy that prioritizes observable and unobservable inputs used to measure fair value into three levels: Level 1 – quoted prices (unadjusted) in active markets that are accessible at the measurement date for assets or liabilities; Level 2 – observable prices that are based on inputs not quoted in active markets, but corroborated by market data; and Level 3 – unobservable inputs that are used when little or no market data is available. The fair value hierarchy gives the highest priority to Level 1 inputs and the lowest priority to Level 3 inputs. In determining fair value, we utilize valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs to the extent possible, as well as consider counterparty credit risk in our assessment of fair value.