Significant Accounting Policies (Policies)	9 Months Ended
Sep. 30, 2015
Significant Accounting Policy [Abstract]
Basis of Accounting [Policy Text Block]	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, 2014, 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, 2015 are not necessarily indicative of the operating results for the full year. We currently operate in one business segment.
Recently Issued Accounting Literature [Policy Text Block]	In April 2014, the Financial Accounting Standards Board (“FASB”) issued an update (“ASU 2014-08”) Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity to Accounting Standards Codification (“ASC”) Topic 205, Presentation of Financial Statements and ASC Topic 360, Property Plant and Equipment. Under ASU 2014-08, only disposals that represent a strategic shift that has (or will have) a major effect on the entity's results and operations qualify as discontinued operations. In addition, ASU 2014-08 expands the disclosure requirements for disposals that meet the definition of a discontinued operation and requires entities to disclose information about disposals of individually significant components that do not meet the definition of discontinued operations. ASU 2014-08 was effective for interim and annual reporting periods in fiscal years that begin after December 15, 2014. The adoption of this update on January 1, 2015 did not have any impact on our consolidated financial statements. In May 2014, the FASB issued an update (“ASU 2014-09”) establishing ASC Topic 606 Revenue from Contracts with Customers. ASU 2014-09 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. ASU 2014-09 requires an entity to recognize revenue when it transfers 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. ASU 2014-09 is effective for interim and annual reporting periods in fiscal years that begin after December 15, 2017. We are currently evaluating the impact of the adoption of ASU 2014-09 on our consolidated financial statements. In April 2015, the FASB issued an update (“ASU 2015-03”) Simplifying the Presentation of Debt Issuance Costs to ASC Topic 835, Interest. ASU 2015-03 requires that debt issuance costs be presented in the balance sheet as a direct deduction from the carrying amount of the debt liability to which they relate, consistent with debt discounts, as opposed to being presented as assets. ASU 2015-03 is effective for interim and annual reporting periods in fiscal years that begin after December 15, 2015. The adoption of this update on January 1, 2016 will not have a material impact on our consolidated financial statements.
Marketable Securities, Available-for-sale Securities, Policy [Policy Text Block]	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 income.”
Fair Value Measurement Policy [Policy Text Block]	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.
Share-based Compensation, Option and Incentive Plans Policy [Policy Text Block]	We account for stock-based compensation in accordance with ASC 718, Compensation – Stock Compensation. Our Omnibus Stock Plan provides for grants of incentive and non-qualified stock options, restricted stock, stock appreciation rights, deferred stock units (“DSUs”) and performance shares, as defined, to the directors, officers and employees of the Company and Vornado.
Real Estate [Policy Text Block]	Development and Construction in Progress – We are constructing an apartment tower, The Alexander, above our Rego Park II shopping center, containing 312 units aggregating 255,000 square feet. The estimated cost of this project, including initial leasing costs to achieve stabilized occupancy, is approximately $125,000,000, of which $113,000,000 has been incurred as of September 30, 2015. We capitalize all property operating expenses directly associated with and attributable to, the development and construction of a project, including interest expense. The capitalization period begins when development activities are underway and ends when it is determined that the asset is substantially complete and ready for its intended use, which is typically evidenced by the receipt of a temporary certificate of occupancy (“TCO”). In the third quarter of 2015, we received a TCO for certain floors of the apartment tower where construction has been substantially completed. Accordingly, this portion of the apartment tower has been placed in service. Construction of the remaining floors is expected to be substantially completed during the fourth quarter of 2015 and we expect to receive the related TCO by December 31, 2015.

Significant Accounting Policies (Policies)

9 Months Ended

Sep. 30, 2015

Significant Accounting Policy [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, 2014, 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, 2015 are not necessarily indicative of the operating results for the full year.

We currently operate in one business segment.

Recently Issued Accounting Literature [Policy Text Block]

In April 2014, the Financial Accounting Standards Board (“FASB”) issued an update (“ASU 2014-08”) Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity to Accounting Standards Codification (“ASC”) Topic 205, Presentation of Financial Statements and ASC Topic 360, Property Plant and Equipment. Under ASU 2014-08, only disposals that represent a strategic shift that has (or will have) a major effect on the entity's results and operations qualify as discontinued operations. In addition, ASU 2014-08 expands the disclosure requirements for disposals that meet the definition of a discontinued operation and requires entities to disclose information about disposals of individually significant components that do not meet the definition of discontinued operations. ASU 2014-08 was effective for interim and annual reporting periods in fiscal years that begin after December 15, 2014. The adoption of this update on January 1, 2015 did not have any impact on our consolidated financial statements.

In May 2014, the FASB issued an update (“ASU 2014-09”) establishing ASC Topic 606 Revenue from Contracts with Customers. ASU 2014-09 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. ASU 2014-09 requires an entity to recognize revenue when it transfers 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. ASU 2014-09 is effective for interim and annual reporting periods in fiscal years that begin after December 15, 2017. We are currently evaluating the impact of the adoption of ASU 2014-09 on our consolidated financial statements.

In April 2015, the FASB issued an update (“ASU 2015-03”) Simplifying the Presentation of Debt Issuance Costs to ASC Topic 835, Interest. ASU 2015-03 requires that debt issuance costs be presented in the balance sheet as a direct deduction from the carrying amount of the debt liability to which they relate, consistent with debt discounts, as opposed to being presented as assets. ASU 2015-03 is effective for interim and annual reporting periods in fiscal years that begin after December 15, 2015. The adoption of this update on January 1, 2016 will not have a material impact on our consolidated financial statements.

Marketable Securities, Available-for-sale Securities, Policy [Policy Text Block]

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 income.”

Fair Value Measurement Policy [Policy Text Block]

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.

Share-based Compensation, Option and Incentive Plans Policy [Policy Text Block]

We account for stock-based compensation in accordance with ASC 718, Compensation – Stock Compensation. Our Omnibus Stock Plan provides for grants of incentive and non-qualified stock options, restricted stock, stock appreciation rights, deferred stock units (“DSUs”) and performance shares, as defined, to the directors, officers and employees of the Company and Vornado.

Real Estate [Policy Text Block]

Development and Construction in Progress – We are constructing an apartment tower, The Alexander, above our Rego Park II shopping center, containing 312 units aggregating 255,000 square feet. The estimated cost of this project, including initial leasing costs to achieve stabilized occupancy, is approximately $125,000,000, of which $113,000,000 has been incurred as of September 30, 2015. We capitalize all property operating expenses directly associated with and attributable to, the development and construction of a project, including interest expense. The capitalization period begins when development activities are underway and ends when it is determined that the asset is substantially complete and ready for its intended use, which is typically evidenced by the receipt of a temporary certificate of occupancy (“TCO”). In the third quarter of 2015, we received a TCO for certain floors of the apartment tower where construction has been substantially completed. Accordingly, this portion of the apartment tower has been placed in service. Construction of the remaining floors is expected to be substantially completed during the fourth quarter of 2015 and we expect to receive the related TCO by December 31, 2015.