Term used as the basis for determining the amortization for a debt instrument

No definition available.

Name:	alx_DebtAmortizationPeriod
Namespace Prefix:	alx_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt instrument maturity date

+ References

No definition available.

+ Details

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Namespace Prefix:	alx_
Data Type:	us-types:dateStringItemType
Balance Type:	na
Period Type:	instant

- Definition

Intital term to maturity for a debt instrument

+ References

No definition available.

+ Details

Name:	alx_DebtTerm
Namespace Prefix:	alx_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- 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

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 SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

Reference 2: 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 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 maturities after year five following the date of the latest balance sheet presented in the financial statements, which may include maturities of long-term debt, sinking fund requirements, and other securities redeemable at fixed of determinable prices and dates.

+ References

Reference 1: 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.

Reference 2: 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

+ 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 maturing within the next twelve months following the date of the latest balance sheet presented in the financial statements, which may include maturities of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates.

+ References

+ Details

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

- Definition

Amount of long-term debt maturing in year five following the date of the latest balance sheet presented in the financial statements, which may include maturities of long-term debt, sinking fund requirements, and other securities redeemable at fixed of determinable prices and dates.

+ 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 maturing in year four following the date of the latest balance sheet presented in the financial statements, which may include maturities of long-term debt, sinking fund requirements, and other securities redeemable at fixed of determinable prices and dates.

+ 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 maturing in year three following the date of the latest balance sheet presented in the financial statements, which may include maturities of long-term debt, sinking fund requirements, and other securities redeemable at fixed of determinable prices and dates.

+ 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 maturing in year two following the date of the latest balance sheet presented in the financial statements, which may include maturities of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates.

+ References

+ Details

Name:	us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo
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, carrying value as of the balance sheet date of all notes and loans payable (with maturities initially due after one year or beyond the operating cycle if longer).

+ References

Reference 1: 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 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 7

-Section 03

-Paragraph 16

Reference 3: 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

+ Details

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

- Definition

The cash inflow from a debt initially having maturity due after one year or beyond the operating cycle, if longer.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 14

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3255-108585

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 18

-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.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 19

-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.

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Financing Activities

-URI http://asc.fasb.org/extlink&oid=6513228

+ Details

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

- Definition

The net cash inflow or outflow in aggregate debt due to repayments and proceeds from additional borrowings.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 18

-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.

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Financing Activities

-URI http://asc.fasb.org/extlink&oid=6513228

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	debit
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- Details

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Notes and Mortgages Payable (Details) (USD $)	12 Months Ended					0 Months Ended			12 Months Ended			12 Months Ended		0 Months Ended				0 Months Ended
Notes and Mortgages Payable (Details) (USD $)	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Rego Park 1 Shopping [Member] Mortgages [Member] Secured [Member]	Dec. 31, 2010 Rego Park 1 Shopping [Member] Mortgages [Member] Secured [Member]	Nov. 30, 2011 Rego Park 2 Property [Member] Mortgages [Member] Secured [Member]	Dec. 31, 2011 Rego Park 2 Property [Member] Mortgages [Member] Secured [Member]	Dec. 31, 2010 Rego Park 2 Property [Member] Mortgages [Member] Secured [Member]	Dec. 31, 2011 Lexington Avenue Property [Member]	Dec. 31, 2011 Lexington Avenue Property [Member] Mortgages [Member] Secured [Member] Office Space [Member]	Dec. 31, 2010 Lexington Avenue Property [Member] Mortgages [Member] Secured [Member] Office Space [Member]	Dec. 31, 2011 Lexington Avenue Property [Member] Mortgages [Member] Secured [Member] Retail Space [Member]	Dec. 31, 2010 Lexington Avenue Property [Member] Mortgages [Member] Secured [Member] Retail Space [Member]	Jun. 10, 2011 Kings Plaza Regional Shopping Center [Member] Mortgages [Member] Secured [Member]	Dec. 31, 2011 Kings Plaza Regional Shopping Center [Member] Mortgages [Member] Secured [Member]	Dec. 31, 2010 Kings Plaza Regional Shopping Center [Member] Mortgages [Member] Secured [Member]	Dec. 31, 2011 Paramus Property [Member]	Oct. 05, 2011 Paramus Property [Member] Mortgages [Member] Secured [Member]	Dec. 31, 2011 Paramus Property [Member] Mortgages [Member] Secured [Member]	Dec. 31, 2010 Paramus Property [Member] Mortgages [Member] Secured [Member]
Mortgage Loans on Real Estate [Line Items]
Debt Instrument Maturity Date String				2012-03			2018-11			2014-02		2015-07			2016-06				2018-10
Interest rate				0.75%			2.15%			5.33%		4.93%			2.24%		2.90%		2.90%
Balance	$ 1,330,932,000	$ 1,246,411,000		$ 78,246,000	$ 78,246,000		$ 274,796,000	$ 277,200,000	$ 320,000,000	$ 339,890,000	$ 351,751,000	$ 320,000,000	$ 320,000,000		$ 250,000,000	$ 151,214,000	$ 68,000,000.00		$ 68,000,000	$ 68,000,000
Basis spread over LIBOR						1.85%								1.70%
Mortgage Loan Converted To Recourse									75,000,000			75,000,000
Proceeds from Issuance of Long-term Debt	593,000,000	34,828,000	162,961,000			275,000,000								250,000,000
Proceeds from (Repayments of) Debt														95,000,000
Debt amortization period						30 years
Debt Term						7 years								5 years				7 years
Net carrying value of real estate collaterizing the debt	873,911,000
Repayments of Long-term Debt [Abstract]
2012	93,262,000
2013	15,957,000
2014	317,179,000
2015	323,192,000
2016	253,440,000
Thereafter	$ 327,902,000