Summary of Significant Accounting Policies (Policies)	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Dec. 31, 2017
Accounting Policies [Abstract]
Basis of Presentation	The accompanying consolidated financial statements include our accounts and those of our consolidated subsidiaries. All intercompany amounts have been eliminated. Our financial statements are prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”), which requires us 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 financial statements and the reported amounts of revenues and expenses during the reporting periods. Actual results could differ from those estimates. Certain prior year balances have been reclassified in order to conform to current year presentation.
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 (“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 adopted this standard effective January 1, 2018 using the modified retrospective approach, which requires applying the new standard to all existing contracts not yet completed as of the effective date and recording a cumulative-effect adjustment to retained earnings as of the beginning of the fiscal year of adoption. We have completed our evaluation of the standard’s impact on our revenue streams. The adoption of this standard is not expected to have a material impact 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 (“ASC 825”). 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. We adopted this standard effective January 1, 2018 using the modified retrospective approach. 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 (loss) income.” As a result, on January 1, 2018 we recorded an increase to retained earnings of $5,156,000 to recognize the unrealized gains previously recorded within “accumulated other comprehensive income.” Subsequent changes in the fair value of our marketable securities will be recorded to “interest and other income, net.” 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 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 accounting applied by the lessor is largely unchanged from that applied under the existing lease standard. 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, and will continue to recognize expense on a straight-line basis upon adoption of this standard. ASU 2016-02 is effective for interim and annual reporting periods in fiscal years that begin after December 15, 2018, with early adoption permitted. We will adopt this standard effective January 1, 2019 using the modified retrospective approach and will elect to use the practical expedients provided by this standard. 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 a material impact on our consolidated financial statements. ALEXANDER’S, INC. AND SUBSIDIARIES NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED) 2. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - continued 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 standard did not have a material impact on our consolidated financial statements. 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. 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 on January 1, 2018 is not expected to have an impact on our consolidated financial statements. 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 on January 1, 2019 is not expected to have a material impact on our consolidated financial statements.
Real Estate	Real estate is carried at cost, net of accumulated depreciation and amortization. As of December 31, 2017 and 2016, the carrying amount of our real estate, net of accumulated depreciation and amortization, was $754,324,000 and $780,814,000, respectively. Maintenance and repairs are expensed as incurred. Depreciation requires an estimate by management of the useful life of each property and improvement as well as an allocation of the costs associated with a property to its various components. 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. General and administrative costs are expensed as incurred. ALEXANDER’S, INC. AND SUBSIDIARIES NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED) 2. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - continued Our properties and related intangible assets, including properties to be developed in the future, are individually reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of the assets may not be recoverable. An impairment exists when the carrying amount of an asset exceeds the sum of the undiscounted cash flows expected to result from the use and eventual disposition of the asset. Estimates of future cash flows are based on our current plans, intended holding periods and available market information at the time the analyses are prepared. For our development properties, estimates of future cash flows also include all future expenditures necessary to develop the asset, including interest payments that will be capitalized as part of the cost of the asset. An impairment loss is recognized only if the carrying amount of the asset is not recoverable and is measured based on the excess of the property’s carrying amount over its estimated fair value. If our estimates of future cash flows, anticipated holding periods, or fair values change, based on market conditions or otherwise, our evaluation of impairment charges may be different and such differences could be material to our consolidated financial statements. Estimates of future cash flows are subjective and are based, in part, on assumptions regarding future occupancy, rental rates and capital requirements that could differ materially from actual results. Plans to hold properties over longer periods decrease the likelihood of recording impairment losses.
Cash and Cash Equivalents	Cash and cash equivalents consist of highly liquid investments with original maturities of three months or less and are carried at cost, which approximates fair value, due to their short-term maturities. The majority of our cash and cash equivalents consist of (i) deposits at major commercial banks, which may at times exceed the Federal Deposit Insurance Corporation limit, (ii) United States Treasury Bills, (iii) money market funds, which invest in United States Treasury Bills and (iv) certificates of deposit placed through an account registry service (“CDARS”). To date we have not experienced any losses on our invested cash.
Short-term Investments	Short-term investments consist of United States Treasury Bills with original maturities greater than three but less than six months. These highly liquid investments are classified as available-for-sale and are presented at fair value on our consolidated balance sheets. Prior to January 1, 2018, unrealized gains and losses resulting from these investments were included in “other comprehensive (loss) income.” Effective January 1, 2018, changes in the fair value of these investments are recognized in current period earnings in accordance with ASC 825.
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.
Marketable Securities	Our marketable securities consist of common shares of The Macerich Company (NYSE: MAC) (“Macerich”), which are classified as available-for-sale. Available-for-sale securities are presented at fair value on our consolidated balance sheets. Prior to January 1, 2018, unrealized gains and losses resulting from the mark-to-market of these securities were included in “other comprehensive (loss) income.” Effective January 1, 2018, changes in the fair value of these securities are recognized in current period earnings in accordance with ASC 825. We evaluate our marketable securities for impairment at the end of each reporting period. If investments have unrealized losses, we evaluate the underlying cause of the decline in value and the estimated recovery period, as well as the severity and duration of the decline. In our evaluation, we consider our ability and intent to hold our investment for a reasonable period of time sufficient for us to recover our cost basis, as well as the near-term prospects for the investment in relation to the severity and duration of the decline.
Allowance for Doubtful Accounts	We periodically evaluate the collectibility of amounts due from tenants, including the receivable arising from the straight-lining of rents, and maintain an allowance for doubtful accounts ($1,501,000 and $1,473,000 as of December 31, 2017 and 2016, respectively) for estimated losses resulting from the inability of tenants to make required payments under the lease agreements. We exercise judgment in establishing these allowances and consider payment history and current credit status in developing these estimates.
Deferred Charges	Direct financing costs are deferred and amortized over the terms of the related agreements as a component of interest and debt expense. Direct costs related to leasing activities are capitalized and amortized on a straight-line basis over the lives of the related leases. All other deferred charges are amortized on a straight-line basis, which approximates the effective interest rate method, in accordance with the terms of the agreements to which they relate.
Revenue Recognition	We have the following revenue sources and revenue recognition policies: Base Rent – revenue arising from tenant leases. These rents are recognized over the non-cancelable term of the related leases on a straight-line basis, which includes the effects of rent steps and free rent abatements under the leases. We commence rental revenue recognition when the tenant takes possession of the leased space and the leased space is substantially ready for its intended use. In addition, in circumstances where we provide a tenant improvement allowance for improvements that are owned by the tenant, we recognize the allowance as a reduction of rental revenue on a straight-line basis over the term of the lease. Percentage Rent – revenue arising from retail tenant leases that is contingent upon the sales of tenants exceeding defined thresholds. These rents are recognized only after the contingency has been removed (i.e., when tenant sales thresholds have been achieved). Expense Reimbursements – revenue arising from tenant leases which provide for the recovery of all or a portion of the operating expenses and real estate taxes of the respective properties. This revenue is recognized in the same periods as the expenses are incurred. Parking Income – revenue arising from the rental of parking space at our properties. This income is recognized as the service is provided.
Income Taxes	We operate in a manner intended to enable us to continue to qualify as a REIT under Sections 856 – 860 of the Internal Revenue Code of 1986, as amended (the “Code”). In order to maintain our qualification as a REIT under the Code, we must distribute at least 90% of our taxable income to stockholders each year. We distribute to our stockholders 100% of our taxable income and therefore, no provision for Federal income taxes is required. Dividends distributed for the year ended December 31, 2017 were characterized, for federal income taxes, as 99.5% ordinary income and 0.5% long-term capital gain income. Dividends distributed for the year ended December 31, 2016 were characterized, for federal income taxes, as 97.7% ordinary income and 2.3% long-term capital gain income. Dividends distributed for the year ended December 31, 2015 were categorized, for federal income tax purposes, as 97.3% ordinary income and 2.7% long-term capital gain income.