Summary of Significant Accounting Policies (Policies)	3 Months Ended
Mar. 31, 2016
Summary Of Significant Accounting Policies [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, 2015, 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 months ended March 31, 2016 are not necessarily indicative of the operating results for the full year. We currently operate in one business segment.
Recently Issued Accounting Literature	4. Recently Issued Accounting Literature 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. 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 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, including the requirement to measure certain equity investments at fair value with changes in fair value recognized in net income. ASU 2016-01 is effective for interim and annual reporting periods in fiscal years beginning after December 15, 2017. We are currently evaluating the impact of the adoption of ASU 2016-01 on our consolidated financial statements. In February 2016, the FASB issued (“ASU 2016-02”) 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. The new standard requires lessors to account for leases using an approach that is substantially equivalent to existing guidance. ASU 2016-02 is effective for reporting periods beginning after December 15, 2018, with early adoption permitted. We are currently evaluating the impact of the adoption of ASU 2016-02 on our consolidated financial statements. 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. 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 is effective for interim and annual reporting periods in fiscal years beginning after December 15, 2017. We are currently evaluating the impact of the adoption of ASU 2016-09 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 income.”
Fair Value Measurement	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.

Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2016

Summary Of Significant Accounting Policies [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, 2015, 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 months ended March 31, 2016 are not necessarily indicative of the operating results for the full year.

We currently operate in one business segment.

Recently Issued Accounting Literature

4. Recently Issued Accounting Literature

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. 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 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, including the requirement to measure certain equity investments at fair value with changes in fair value recognized in net income. ASU 2016-01 is effective for interim and annual reporting periods in fiscal years beginning after December 15, 2017. We are currently evaluating the impact of the adoption of ASU 2016-01 on our consolidated financial statements.

In February 2016, the FASB issued (“ASU 2016-02”) 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. The new standard requires lessors to account for leases using an approach that is substantially equivalent to existing guidance. ASU 2016-02 is effective for reporting periods beginning after December 15, 2018, with early adoption permitted. We are currently evaluating the impact of the adoption of ASU 2016-02 on our consolidated financial statements.

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. 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 is effective for interim and annual reporting periods in fiscal years beginning after December 15, 2017. We are currently evaluating the impact of the adoption of ASU 2016-09 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 income.”

Fair Value Measurement

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.