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

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

- Definition

The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

+ References

No definition available.

+ Details

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

- Definition

Amount of liabilities classified as other.

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- 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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Period Type:	duration

- Definition

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

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Balance Type:	debit
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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.

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- Details

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Related Party Transactions - Narrative (Details)	3 Months Ended
Related Party Transactions - Narrative (Details)	Mar. 31, 2026 USD ($) $ / ft²	Mar. 31, 2025 USD ($)	Dec. 31, 2025 USD ($)
Related Party Transaction [Line Items]
Fees to related party	$ 2,074,000	$ 2,591,000
Development fees
Related Party Transaction [Line Items]
Fees to related party	65,000	419,000
Leasing fees
Related Party Transaction [Line Items]
Fees to related party	30,000	$ 13,000
Related Party
Related Party Transaction [Line Items]
Management fee agreement value	2,800,000
Other liabilities	$ 0		$ 134,000
Related Party \| Development fees
Related Party Transaction [Line Items]
Development fee as percentage of development costs	6.00%
Other liabilities			34,000
Related Party \| 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 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%
Other liabilities			$ 100,000
Percentage of third-party lease commission	33.333%
Related Party \| Retail Space \| Rego Park II \| Property management, cleaning, engineering, parking and security fees
Related Party Transaction [Line Items]
Property management fee, percent fee	2.00%
Related Party \| Office and Retail Space \| 731 Lexington Avenue \| Property management, cleaning, engineering, parking and security fees
Related Party Transaction [Line Items]
Property management fee agreement, price per square foot \| $ / ft²	0.50
Related Party \| Common Area \| 731 Lexington Avenue \| Property management, cleaning, engineering, parking and security fees
Related Party Transaction [Line Items]
Property management fee escalation percentage per annum	3.00%
Related Party \| Common Area \| 731 Lexington Avenue \| Management And Development Agreement, Base Management Fee
Related Party Transaction [Line Items]
Fees to related party	$ 399,000
Vornado \| Leasing fees
Related Party Transaction [Line Items]
Asset sale commission threshold	$ 50,000,000
Alexander's Inc. \| Related Party \| Vornado
Related Party Transaction [Line Items]
Subsidiary, Ownership Percentage, Parent	32.40%