Mortgages Payable (Details) - USD ($)	6 Months Ended
Jun. 30, 2015	Dec. 31, 2014
Mortgage Loans on Real Estate [Line Items]
Notes Payable (in US dollars)	$ 1,031,213,000	$ 1,032,780,000
Rego Park 1 Shopping [Member] \| Mortgages [Member] \| Secured [Member]
Mortgage Loans on Real Estate [Line Items]
Percentage of cash mortgage collateralized	100.00%
Debt Instrument Maturity Date String	2016-03
Interest rate (in percentage)	0.40%
Notes Payable (in US dollars)	$ 78,246,000	78,246,000
Duration Of Mortgage Loan Extension	1 year
Rego Park 2 Property [Member] \| Mortgages [Member] \| Secured [Member]
Mortgage Loans on Real Estate [Line Items]
Debt Instrument Maturity Date String	2018-11
Interest rate (in percentage)	2.04%
Notes Payable (in US dollars)	$ 264,967,000	266,534,000
Basis spread over LIBOR (in percentage)	1.85%
Lexington Avenue Property [Member] \| Retail Space [Member]
Mortgage Loans on Real Estate [Line Items]
Notes Payable (in US dollars)	$ 320,000,000
Mortgage loan converted to recourse (in US Dollars)	$ 75,000,000
Lexington Avenue Property [Member] \| Mortgages [Member] \| Secured [Member] \| Office Space [Member]
Mortgage Loans on Real Estate [Line Items]
Debt Instrument Maturity Date String	2021-03
Interest rate (in percentage)	1.14%
Notes Payable (in US dollars)	$ 300,000,000	300,000,000
Basis spread over LIBOR (in percentage)	0.95%
Lexington Avenue Property [Member] \| Mortgages [Member] \| Secured [Member] \| Retail Space [Member]
Mortgage Loans on Real Estate [Line Items]
Debt Instrument Maturity Date String	2015-08
Interest rate (in percentage)	4.93%
Notes Payable (in US dollars)	$ 320,000,000	320,000,000
Lexington Avenue Property [Member] \| Mortgages [Member] \| Secured [Member] \| Retail Space [Member] \| Maximum [Member]
Mortgage Loans on Real Estate [Line Items]
Mortgage loan converted to recourse (in US Dollars)	$ 75,000,000
Paramus Property [Member] \| Mortgages [Member] \| Secured [Member]
Mortgage Loans on Real Estate [Line Items]
Debt Instrument Maturity Date String	2018-10
Interest rate (in percentage)	2.90%
Notes Payable (in US dollars)	$ 68,000,000	$ 68,000,000

Debt instrument maturity date

No definition available.

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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

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Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

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Namespace Prefix:	alx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Percentage of cash mortgage collateralized

+ References

No definition available.

+ Details

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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

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- 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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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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- 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

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-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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- Details

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