Debt instrument maturity date

No definition available.

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

- Definition

The term of the extension, in years.

+ References

No definition available.

+ Details

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

- Definition

The term of the extension available, in years.

+ References

No definition available.

+ Details

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

- Definition

The total number of term extensions available to the borrower.

+ References

No definition available.

+ Details

Name:	alx_NumberOfExtensionsAvailable
Namespace Prefix:	alx_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of cash mortgage collateralized

+ References

No definition available.

+ Details

Name:	alx_PercentageOfCashMortgageCollateralized
Namespace Prefix:	alx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

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

- Definition

The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

+ References

No definition available.

+ Details

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

- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

Reference 1: 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

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 SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

+ Details

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

No definition available.

+ Details

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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 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 2: 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 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 13, 16

-Article 9

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 20, 22

-Article 5

+ Details

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

- Details

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Namespace Prefix:
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- 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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- Details

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Data Type:	na
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- Details

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Mortgages Payable (Details)		9 Months Ended
Mortgages Payable (Details)	Aug. 05, 2015 USD ($) extensionoptions	Sep. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Mortgage Loans on Real Estate [Line Items]
Notes Payable (in US dollars)		$ 1,060,408,000	$ 1,032,780,000
Rego Park 1 Shopping [Member] \| Mortgages [Member] \| Secured [Member]
Mortgage Loans on Real Estate [Line Items]
Notes Payable (in US dollars)		$ 78,246,000	78,246,000
Maturity date		2016-03
Percentage of cash mortgage collateralized		100.00%
Interest rate (in percentage)		0.40%
Duration Of Mortgage Loan Extension		1 year
Rego Park 2 Property [Member] \| Mortgages [Member] \| Secured [Member]
Mortgage Loans on Real Estate [Line Items]
Notes Payable (in US dollars)		$ 264,162,000	266,534,000
Basis spread over LIBOR (in percentage)		1.85%
Maturity date		2018-11
Interest rate (in percentage)		2.04%
Lexington Avenue Property [Member] \| Mortgages [Member] \| Secured [Member] \| Office Space [Member]
Mortgage Loans on Real Estate [Line Items]
Notes Payable (in US dollars)		$ 300,000,000	300,000,000
Basis spread over LIBOR (in percentage)		0.95%
Maturity date		2021-03
Interest rate (in percentage)		1.16%
Lexington Avenue Property [Member] \| Mortgages [Member] \| Secured [Member] \| Retail Space [Member]
Mortgage Loans on Real Estate [Line Items]
Notes Payable (in US dollars)	$ 350,000,000	$ 350,000,000	320,000,000
Debt Instrument, Description of Variable Rate Basis	LIBOR
Basis spread over LIBOR (in percentage)	1.40%	1.40%
Maturity date	August 2020	2022-08
Number Of Extensions Available \| extensionoptions	2
Term of extension available	1 year
Interest rate (in percentage)		1.61%
Paramus Property [Member] \| Mortgages [Member] \| Secured [Member]
Mortgage Loans on Real Estate [Line Items]
Notes Payable (in US dollars)		$ 68,000,000	$ 68,000,000
Maturity date		2018-10
Interest rate (in percentage)		2.90%