Asset sale commission threshold.

No definition available.

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- Definition

Percentage of development costs that the related party will receive as per the Development agreement.

+ References

No definition available.

+ Details

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- Definition

Percentage of rent payable to related party for leasing services for the eleventh through twentieth years of the lease.

+ References

No definition available.

+ Details

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- Definition

Percentage of rent payable to related party for leasing services for the first ten years of the lease.

+ References

No definition available.

+ Details

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- Definition

Percentage of rent payable to related party for leasing services for the twenty-first through thirtieth years of the lease.

+ References

No definition available.

+ Details

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- Definition

The amount of management fee paid to related party to manage corporate activities.

+ References

No definition available.

+ Details

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Balance Type:	debit
Period Type:	duration

- Definition

Percentage of gross proceeds over $50,000,000 from the sale of any assets, payable to related party.

+ References

No definition available.

+ Details

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- Definition

Percentage of gross proceeds under $50,000,000 from the sale of any assets, payable to related party.

+ References

No definition available.

+ Details

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- Definition

Percentage of increase in leasing services payable to related party if a real estate broker is used

+ References

No definition available.

+ Details

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Namespace Prefix:	alx_
Data Type:	dtr-types:percentItemType
Balance Type:	na
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- Definition

Property Management Fee Agreement, Price Per Square Foot

+ References

No definition available.

+ Details

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Data Type:	srt-types:perUnitItemType
Balance Type:	na
Period Type:	duration

- Definition

Annual amount paid to related party as property management fees.

+ References

No definition available.

+ Details

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

- Definition

Annual increase in the amount paid to related party as property management fees.

+ References

No definition available.

+ Details

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Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

+ References

No definition available.

+ Details

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Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The amount of cash paid for commissions during the current period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3536-108585

+ Details

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Balance Type:	credit
Period Type:	duration

- Definition

The percentage charged for managing real estate properties.

+ References

No definition available.

+ Details

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Data Type:	dtr-types:percentItemType
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- Definition

Receivables to be collected from (obligations owed to) related parties, net as of the balance sheet date where one party can exercise control or significant influence over another party; including affiliates, owners or officers and their immediate families, pension trusts, and so forth.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Details

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- Details

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Data Type:	na
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- Details

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- Details

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- Details

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- Details

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- Details

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- Details

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- Details

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- Details

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Related Party Transactions - Additional Information (Detail) - Vornado	1 Months Ended	3 Months Ended	9 Months Ended
	Nov. 01, 2021 USD ($)	Jun. 30, 2021 USD ($)	Sep. 30, 2021 USD ($) $ / ft²	Dec. 31, 2020 USD ($)
Related Party Transaction [Line Items]
Management fee agreement value			$ 2,800,000
Payments for commissions		$ 300,000
Subsequent Event
Related Party Transaction [Line Items]
Payments for commissions	$ 750,000
Property Management Fees \| Rego Park II \| Retail Space
Related Party Transaction [Line Items]
Property management fee agreement percentage of income			2.00%
Property Management Fees \| 731 Lexington Avenue \| Office and Retail Space
Related Party Transaction [Line Items]
Property management fee agreement, price per square foot \| $ / ft²			0.50
Property Management Fees \| 731 Lexington Avenue \| Common Area
Related Party Transaction [Line Items]
Property management fee agreement value			$ 344,000
Property management fee escalation percentage per annum			3.00%
Leasing fees
Related Party Transaction [Line Items]
Lease fee percentage of rent one to ten years			3.00%
Lease fee percentage of rent eleven to twenty years			2.00%
Lease fee percentage of rent twenty first to thirty years			1.00%
Percentage increase lease fee if broker used			1.00%
Percentage commissions on sale of assets under fifty million			3.00%
Asset sale commission threshold			$ 50,000,000
Percentage commissions on sale of assets over fifty million			1.00%
Amounts due to related party			$ 1,291,000	$ 114,000
Development fees
Related Party Transaction [Line Items]
Development fee as percentage of development costs			6.00%
Amounts due to related party			$ 109,000	557,000
Management, property management, cleaning, engineering and security fees
Related Party Transaction [Line Items]
Amounts due to related party			$ 692,000	$ 845,000
Vornado \| Alexander's Inc.
Related Party Transaction [Line Items]
Noncontrolling interest, ownership percentage by noncontrolling owners			32.40%