Payments For Derivative Instrument

No definition available.

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

- Definition

The carrying value, as of the date of each statement of financial position presented, of real estate which is owned and pledged as collateral for related debt obligation

+ References

No definition available.

+ Details

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

- 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
-Name Regulation S-X (SX)
-Number 210
-Section 12
-Subsection 29
-Publisher SEC

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-Topic 948
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-Paragraph 1
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479628/948-310-S99-1

+ Details

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Namespace Prefix:	srt_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

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-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1B

+ Details

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

- Definition

Amount of the required periodic payments applied to principal.

+ References

+ Details

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

- Definition

Cap rate on an interest rate derivative such as an interest rate cap or collar. If market rates exceed the cap rate, a payment or receipt is triggered on the contract.

+ References

No definition available.

+ Details

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

- Definition

Amount, after deduction of unamortized premium (discount) and debt issuance cost, of long-term debt. Excludes lease obligation.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section S99
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-URI https://asc.fasb.org/1943274/2147481139/470-20-50-4

+ Details

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

- Definition

Period between issuance and maturity of long-term debt, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)(i)
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+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_MortgagesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=alx_LondonInterbankOfferedRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_PrimeRateMember
Namespace Prefix:
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Period Type:

Mortgages Payable - Narrative (Details) - USD ($)		1 Months Ended
Mortgages Payable - Narrative (Details) - USD ($)	Jun. 11, 2024	Jul. 15, 2023	Jun. 30, 2023	Dec. 31, 2024	Sep. 30, 2024	Sep. 29, 2024	Jun. 09, 2023
Mortgage Loans on Real Estate
Net carrying value of real estate collateralizing the debt				$ 587,548,000
731 Lexington Avenue
Mortgage Loans on Real Estate
Payments for derivative instrument			$ 11,258,000
731 Lexington Avenue \| Mortgages
Mortgage Loans on Real Estate
Mortgage loans, extension option	4 months						1 year
Mortgage loans	$ 490,000,000				$ 400,000,000	$ 490,000,000	$ 500,000,000
Paydown	$ 10,000,000
Fixed interest rate				5.04%
731 Lexington Avenue \| Mortgages \| LIBOR
Mortgage Loans on Real Estate
Rate spread		0.90%
Interest cap rate		6.00%
731 Lexington Avenue \| Mortgages \| Prime Rate
Mortgage Loans on Real Estate
Interest cap rate			6.00%