Name:	alx_AllowanceForDoubtfulAccountsAbstract
Namespace Prefix:	alx_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

The original maximum maturity date of all liquid investments classified as cash and cash equivalents

+ References

No definition available.

+ Details

Name:	alx_CashAndCashEquivalentMaturityMaximum
Namespace Prefix:	alx_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The cumulative difference between book value and tax basis of assets and liabilities

+ References

No definition available.

+ Details

Name:	alx_CumulativeAmountOfTemporaryDifferenceBetweenBookValueAndTaxBasis
Namespace Prefix:	alx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Difference in the amount of gain on sale of real estate between GAAP and Tax

+ References

No definition available.

+ Details

Name:	alx_IncomeTaxReconciliationGainOnSaleDifference
Namespace Prefix:	alx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Income Tax Reconciliation Straight Line Rent Adjustment

+ References

No definition available.

+ Details

Name:	alx_IncomeTaxReconciliationStraightLineRentAdjustment
Namespace Prefix:	alx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Percentage of taxable income distributed to stockholders each year

+ References

No definition available.

+ Details

Name:	alx_InternalTaxableIncomeDistributionPolicy
Namespace Prefix:	alx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of cash mortgage collateralized

+ References

No definition available.

+ Details

Name:	alx_PercentageOfCashMortgageCollateralized
Namespace Prefix:	alx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The percentage of taxable income to be distributed to stockholders each year in order to maintain qualification as REIT under the Code.

+ References

No definition available.

+ Details

Name:	alx_RealEstateInvestmentTrustDistributableIncomePolicy
Namespace Prefix:	alx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	alx_ReconciliationOfNetIncomeAttributableToCommonShareholdersToEstimatedTaxableIncomeTableAbstract
Namespace Prefix:	alx_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Estimated taxable income after reconciliation for tax differences

+ References

No definition available.

+ Details

Name:	alx_TaxableIncomeLoss
Namespace Prefix:	alx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	alx_TaxTreatmentOfDividendLineItems
Namespace Prefix:	alx_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

For an unclassified balance sheet, a valuation allowance for receivables due a company that are expected to be uncollectible.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 4

-Article 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5074-111524

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.4)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_AllowanceForDoubtfulAccountsReceivable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_CashAndCashEquivalentsAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to nondeductible depreciation and amortization.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 12

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32687-109319

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 13

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32698-109319

+ Details

Name:	us-gaap_IncomeTaxReconciliationNondeductibleExpenseDepreciationAndAmortization
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

+ References

+ Details

Name:	us-gaap_IncomeTaxReconciliationOtherAdjustments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 260

-SubTopic 10

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Other Comprehensive Income

-URI http://asc.fasb.org/extlink&oid=6519514

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Net Income

-URI http://asc.fasb.org/extlink&oid=6518256

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.19)

-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.18)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.22)

-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 8: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 19

-Article 5

Reference 9: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 20

-Article 9

+ Details

Name:	us-gaap_NetIncomeLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Describes the periods of time over which an entity anticipates to receive utility from its property, plant and equipment (that is, the periods of time over which an entity allocates the initial cost of its property, plant and equipment).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section 50

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6367646&loc=d3e18780-107790

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 360

-SubTopic 10

-Section 35

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=6539997&loc=d3e1205-110223

+ Details

Name:	us-gaap_PropertyPlantAndEquipmentEstimatedUsefulLives
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_RealEstatePropertiesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of increase (decrease) to previously recorded tax expense. Includes, but is not limited to, significant settlements of income tax disputes, and unusual tax positions or infrequent actions taken by the entity, including tax assessment reversal, and IRS tax settlement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 25

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=29652012&loc=d3e28446-109314

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 25

-Paragraph 10

-URI http://asc.fasb.org/extlink&oid=29652012&loc=d3e28511-109314

+ Details

Name:	us-gaap_TaxAdjustmentsSettlementsAndUnusualProvisions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of expense for interest on an underpayment of income taxes and penalties related to a tax position claimed or expected to be claimed in the tax return.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 15

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32718-109319

+ Details

Name:	us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=alx_RegoPark1ShoppingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Significant Accounting Policies (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash and Cash Equivalents [Abstract]
Cash And Cash Equivalent Maturity Maximum									3 months
Allowance For Doubtful Accounts [Abstract]
Allowance for doubtful accounts (in US dollars)	$ 1,544				$ 1,993				$ 1,544	$ 1,993
Tax Treatment Of Dividend [Line Items]
Real Estate Investment Trust Distributable Income Policy (in percentage)									90.00%
Internal Taxable Income Distribution Policy (in percentage)									100.00%
Differences Between Book and Tax Basis	204,815								204,815
Reconciliation of Net Income to Estimated Taxable Income [Abstract]
Net income attributable to Alexander's	18,161us-gaap_NetIncomeLoss	17,692us-gaap_NetIncomeLoss	16,828us-gaap_NetIncomeLoss	15,244us-gaap_NetIncomeLoss	15,790us-gaap_NetIncomeLoss	13,824us-gaap_NetIncomeLoss	13,139us-gaap_NetIncomeLoss	14,162us-gaap_NetIncomeLoss	67,925us-gaap_NetIncomeLoss	56,915us-gaap_NetIncomeLoss	674,387us-gaap_NetIncomeLoss
Straight-line rent adjustments									(2,538)	(3,707)	(4,475)
Depreciation and amortization timing differences									2,283	2,134	910
Interest expense									10	27	29
Reversal of income tax liability									(420)	(206)	0
Additional tax gain on sale of the Kings Plaza Regional Shopping Center									(357)	0	23,928
Other									1,112	(2,213)	4,396
Estimated taxable income									$ 68,015alx_TaxableIncomeLoss	$ 52,950alx_TaxableIncomeLoss	$ 699,175alx_TaxableIncomeLoss
Maximum [Member]
Real Estate Properties [Line Items]
Property plant and equipment estimated useful lives									40 Years
Minimum [Member]
Real Estate Properties [Line Items]
Property plant and equipment estimated useful lives									3 Years
Rego Park I [Member]
Real Estate Properties [Line Items]
Percentage of cash mortgage collateralized	100.00%								100.00%