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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Period Type:	duration

- Definition

Lease Commission, Percentage

+ 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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Period Type:	duration

- 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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Data Type:	xbrli:monetaryItemType
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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Balance Type:	na
Period Type:	duration

- 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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Balance Type:	na
Period Type:	duration

- 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-types1:percentItemType
Balance Type:	na
Period Type:	duration

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

+ References

No definition available.

+ Details

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Data Type:	dtr-types1: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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Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of liabilities classified as other.

+ References

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+ Details

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

- Definition

The percentage charged for managing real estate properties.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of transactions with related party during the financial reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 850
-SubTopic 10
-Name Accounting Standards Codification
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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
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- Details

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- Details

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- Details

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- Details

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Balance Type:
Period Type:

- 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 (Details)		3 Months Ended
	May 01, 2024	Mar. 31, 2024 USD ($) $ / ft²	Mar. 31, 2023 USD ($)	Dec. 31, 2023 USD ($)
Related Party Transaction [Line Items]
Fees to related party		$ 2,389,000	$ 2,150,000
Property Management Fees
Related Party Transaction [Line Items]
Fees to related party		1,636,000	1,409,000
Development fees
Related Party Transaction [Line Items]
Fees to related party		15,000	0
Leasing fees
Related Party Transaction [Line Items]
Fees to related party		38,000	$ 41,000
Vornado
Related Party Transaction [Line Items]
Management fee agreement value		$ 2,800,000
Vornado \| Property Management Fees \| Rego Park II \| Retail Space
Related Party Transaction [Line Items]
Property management fee, percent fee		2.00%
Vornado \| 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
Vornado \| Property Management Fees \| 731 Lexington Avenue \| Common Area
Related Party Transaction [Line Items]
Property management fee escalation percentage per annum		3.00%
Vornado \| Development fees
Related Party Transaction [Line Items]
Development fee as percentage of development costs		6.00%
Vornado \| 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%
Vornado \| Leasing fees \| Subsequent Event
Related Party Transaction [Line Items]
Percentage of third-party lease commission	33.00%
Related Party
Related Party Transaction [Line Items]
Other liabilities		$ 493,000		$ 715,000
Related Party \| Development fees
Related Party Transaction [Line Items]
Other liabilities		14,000
Related Party \| Leasing fees
Related Party Transaction [Line Items]
Other liabilities		38,000		69,000
Related Party \| Management, property management, cleaning, engineering and security fees
Related Party Transaction [Line Items]
Other liabilities		441,000		$ 646,000
Related Party \| Management And Development Agreement, Base Management Fee \| 731 Lexington Avenue \| Common Area
Related Party Transaction [Line Items]
Fees to related party		$ 376,000
Alexander's Inc. \| Vornado \| Vornado
Related Party Transaction [Line Items]
Ownership percentage by noncontrolling owners		32.40%