Special dividend paid to distribute the long term capital gain from the sale of real estate

No definition available.

Name:	alx_CommonStockDividendsDeclaredLongTermCapital
Namespace Prefix:	alx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Special long term capital gain dividend per share to distribute the tax gain resulting from sale of property.

+ References

No definition available.

+ Details

Name:	alx_CommonStockDividendsPerShareDeclaredLongTermCapital
Namespace Prefix:	alx_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Value of common shares received from sale of real estate.

+ References

No definition available.

+ Details

Name:	alx_CommonStockFromSaleOfRealEstate
Namespace Prefix:	alx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	alx_DescriptionOfTransactionAbstract
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- Definition

Total expenses related to discontinued operations

+ References

No definition available.

+ Details

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Namespace Prefix:	alx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

This element represents the net proceeds from the sale of the property (including all forms of consideration received) after repaying the loan and closing costs.

+ References

No definition available.

+ Details

Name:	alx_DisposalGroupProceedsFromSaleOfRealEstate
Namespace Prefix:	alx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Gross sales price for the sale of real estate.

+ References

No definition available.

+ Details

Name:	alx_RealEstatePropertySoldGross
Namespace Prefix:	alx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The element represents the remaining equity interest percentage to be acquired.

+ References

No definition available.

+ Details

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

- Definition

Gain on sale of real estate including portion recognized in income and portion deferred

+ References

No definition available.

+ Details

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

- Definition

Costs of sales and operating expenses for the period incurred from transactions with related parties.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.2)

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

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of gain (loss), after tax expense or benefit and not previously recognized, resulting from the sale of a business component.

+ References

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The excess amount received or due over net assets in a transaction accounted for as a divestiture where a subsidiary, business or operating assets are "sold" by the entity to a newly formed, thinly capitalized, highly leveraged buyer. This gain is not yet recognized and is disclosed on the balance sheet as an offset against the carrying amount of the securities received.

+ References

No definition available.

+ Details

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

- Definition

Amount of operating income or loss attributable to the disposal group, including a component of the entity (discontinued operation), during the reporting period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 205

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6360339&loc=d3e1361-107760

+ Details

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

- Definition

Amount of sales or other form of revenues attributable to the disposal group, including a component of the entity (discontinued operation), during the reporting period.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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Period Type:	duration

- Details

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- Definition

Amount of income (loss) from a disposal group, net of income tax before extraordinary items allocable to noncontrolling interests. Includes, net of tax, income (loss) from operations during the phase-out period, gain (loss) on disposal, provision (or any reversals of earlier provisions) for loss on disposal, and adjustments of a prior period gain (loss) on disposal.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 205

-SubTopic 20

-Section 45

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6892542&loc=d3e957-107759

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.12)

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

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 13

-Article 7

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 205

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6360339&loc=d3e1361-107760

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.14)

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

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 15

-Article 5

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

- Definition

Interest and debt related expenses associated with nonoperating financing activities of the entity.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6450988&loc=d3e26243-108391

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

- Definition

The amount of interest income and other income recognized during the period. Included in this element is interest derived from investments in debt securities, cash and cash equivalents, and other investments which reflect the time value of money or transactions in which the payments are for the use or forbearance of money and other income from ancillary business-related activities (that is, excluding major activities considered part of the normal operations of the business).

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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- Definition

The cash outflow associated with the acquisition of or advances to an entity that is related to it but not strictly controlled (for example, an unconsolidated subsidiary, affiliate, and joint venture or equity method investment) or the acquisition of an additional interest in a subsidiary (controlled entity).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 13

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3213-108585

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

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Discontinued Operations (Details) (USD $)	3 Months Ended	12 Months Ended			1 Months Ended	0 Months Ended		3 Months Ended
Discontinued Operations (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Nov. 30, 2012	Nov. 30, 2012	Nov. 28, 2012	Dec. 31, 2012
Description of Transaction
Common Stock From Sale of Real Estate		$ 0alx_CommonStockFromSaleOfRealEstate	$ 0alx_CommonStockFromSaleOfRealEstate	$ 30,000,000alx_CommonStockFromSaleOfRealEstate
Acquisition of the noncontrolling interest		0	0	7,800,000
Special long-term capital gain dividend				623,178,000
Disposal Group Income And Expenses
Interest and other income, net		2,434,000us-gaap_InterestAndOtherIncome	1,527,000us-gaap_InterestAndOtherIncome	177,000us-gaap_InterestAndOtherIncome
Interest and debt expense		(32,068,000)us-gaap_InterestAndDebtExpense	(44,540,000)us-gaap_InterestAndDebtExpense	(45,652,000)us-gaap_InterestAndDebtExpense
Net gain on sale of real estate	599,628,000		0	599,628,000
Income from discontinued operations		529,000	2,252,000	624,952,000
Fees to Vornado		4,516,000us-gaap_CostsAndExpensesRelatedParty	4,196,000us-gaap_CostsAndExpensesRelatedParty	4,318,000us-gaap_CostsAndExpensesRelatedParty
Kings Plaza Energy Joint Venture [Member]
Description of Transaction
Remaining Equity Interest Percentage Acquired					75.00%	75.00%
Acquisition of the noncontrolling interest					7,800,000
Kings Plaza Regional Shopping Center [Member]
Description of Transaction
Financial Statement Gain							601,976,000
Disposal Group, Deferred Gain on Disposal							2,348,000
Special long-term capital gain dividend (in dollars per share) per share						$ 122.00
Special long-term capital gain dividend						623,178,000
Disposal Group Income And Expenses
Total revenues		0	0	61,836,000
Total expenses		0	0	31,214,000
Operating income from discontinued operations		0	0	30,622,000
Interest and other income, net		529,000	2,252,000	45,000
Interest and debt expense		0	0	(5,343,000)
Net gain on sale of real estate		0	0	599,628,000				599,628,000
Income from discontinued operations		529,000	2,252,000	624,952,000
Fees to Vornado				1,368,000
Kings Plaza Regional Shopping Center [Member] \| Macerich [Member]
Description of Transaction
Real Estate Property Sold Gross							751,000,000
Proceeds from sale of real estate							479,000,000
Common Stock From Sale of Real Estate							$ 30,000,000