Debt instrument maturity date

No definition available.

Name:	alx_DebtInstrumentMaturityDateString
Namespace Prefix:	alx_
Data Type:	invest:dateStringItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of loan that will become recourse in case of a substantial casualty

+ References

No definition available.

+ Details

Name:	alx_MortgageLoanConvertedToRecourse
Namespace Prefix:	alx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_PledgedAssetsSeperatelyReportedRealEstatePledgedAsCollateralAtCarryingValue
Namespace Prefix:	alx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Refinancing Of Mortgage Loan Payable Under Negotiation

+ References

No definition available.

+ Details

Name:	alx_RefinancingOfMortgageLoanPayableUnderNegotiation
Namespace Prefix:	alx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The effective interest rate at the end of the reporting period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(a)(1))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing after the fifth fiscal year following the latest fiscal year.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 470

-SubTopic 10

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 47

-Paragraph 10

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

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

- Definition

Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing in the next fiscal year following the latest fiscal year.

+ References

+ Details

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

- Definition

Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing in the fifth fiscal year following the latest fiscal year.

+ References

+ Details

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

- Definition

Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing in the fourth fiscal year following the latest fiscal year.

+ References

+ Details

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

- Definition

Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing in the third fiscal year following the latest fiscal year.

+ References

+ Details

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

- Definition

Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing in the second fiscal year following the latest fiscal year.

+ References

+ Details

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

- Details

Name:	us-gaap_MortgageLoansOnRealEstateLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Including the current and noncurrent portions, aggregate carrying amount of all types of notes payable, as of the balance sheet date, with initial maturities beyond one year or beyond the normal operating cycle, if longer.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.16)

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.16)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 20, 22

-Article 5

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 13, 16

-Article 9

+ Details

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

- Details

Name:	us-gaap_RepaymentsOfLongTermDebtAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Mortgages Payable (Details) (USD $)							12 Months Ended					12 Months Ended
Mortgages Payable (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013 Rego Park 1 Shopping [Member] Mortgages [Member] Secured [Member]	Dec. 31, 2012 Rego Park 1 Shopping [Member] Mortgages [Member] Secured [Member]	Dec. 31, 2013 Rego Park 2 Property [Member] Mortgages [Member] Secured [Member]	Dec. 31, 2012 Rego Park 2 Property [Member] Mortgages [Member] Secured [Member]	Dec. 31, 2013 Lexington Avenue Property [Member]	Dec. 31, 2013 Lexington Avenue Property [Member] Mortgages [Member] Secured [Member] Office Space [Member]	Dec. 31, 2012 Lexington Avenue Property [Member] Mortgages [Member] Secured [Member] Office Space [Member]	Dec. 31, 2013 Lexington Avenue Property [Member] Mortgages [Member] Secured [Member] Retail Space [Member]	Dec. 31, 2012 Lexington Avenue Property [Member] Mortgages [Member] Secured [Member] Retail Space [Member]	Dec. 31, 2013 Lexington Avenue Property [Member] Mortgages [Member] Secured [Member] Retail Space [Member] Maximum [Member]	Dec. 31, 2013 Paramus Property [Member] Mortgages [Member] Secured [Member]	Dec. 31, 2012 Paramus Property [Member] Mortgages [Member] Secured [Member]
Mortgage Loans on Real Estate [Line Items]
Debt Instrument Maturity Date String			2015-03		2018-11			2014-02		2015-07			2018-10
Interest rate (in percentage)			0.40%		2.02%			5.33%		4.93%			2.90%
Balance	$ 1,049,959,000	$ 1,065,916,000	$ 78,246,000	$ 78,246,000	$ 269,496,000	$ 272,245,000	$ 320,000,000	$ 314,217,000	$ 327,425,000	$ 320,000,000	$ 320,000,000		$ 68,000,000	$ 68,000,000
Basis spread over LIBOR (in percentage)					1.85%
Mortgage Loan Converted To Recourse							75,000,000					75,000,000
Refinancing Of Mortgage Loan Payable Under Negotiation							300,000,000
Net carrying value of real estate collaterizing the debt	728,007,000
Repayments of Long-term Debt [Abstract]
2014	317,179,000
2015	401,439,000
2016	3,440,000
2017	3,707,000
2018	324,194,000
Thereafter	$ 0