Debt instrument maturity date

No definition available.

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

- 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

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 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

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 12

-Section 13

-Sentence Column B

+ Details

Name:	invest_DerivativeNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
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 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

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

- 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:	num:percentItemType
Balance Type:	na
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 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 SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 20, 22

-Article 5

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

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

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

- 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 Accounting Standards Codification

-Glossary Financing Activities

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

+ Details

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

Mortgages Payable (Details) (USD $)					3 Months Ended		3 Months Ended	0 Months Ended					3 Months Ended
Mortgages Payable (Details) (USD $)	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014 Rego Park 1 Shopping [Member] Mortgages [Member] Secured [Member]	Dec. 31, 2013 Rego Park 1 Shopping [Member] Mortgages [Member] Secured [Member]	Mar. 31, 2014 Rego Park 2 Property [Member] Mortgages [Member] Secured [Member]	Dec. 31, 2013 Rego Park 2 Property [Member] Mortgages [Member] Secured [Member]	Mar. 31, 2014 Lexington Avenue Property [Member]	Feb. 28, 2014 Lexington Avenue Property [Member] Mortgages [Member] Secured [Member] Office Space [Member] Extensions	Mar. 31, 2014 Lexington Avenue Property [Member] Mortgages [Member] Secured [Member] Office Space [Member]	Dec. 31, 2013 Lexington Avenue Property [Member] Mortgages [Member] Secured [Member] Office Space [Member]	Mar. 31, 2014 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]	Mar. 31, 2014 Lexington Avenue Property [Member] Mortgages [Member] Secured [Member] Retail Space [Member] Maximum [Member]	Mar. 31, 2014 Paramus Property [Member] Mortgages [Member] Secured [Member]	Dec. 31, 2013 Paramus Property [Member] Mortgages [Member] Secured [Member]
Mortgage Loans on Real Estate [Line Items]
Percentage of cash mortgage collateralized			100.00%
Debt Instrument Maturity Date String			2015-03		2018-11				2021-03		2015-07			2018-10
Interest rate (in percentage)			0.40%		2.00%				1.11%		4.93%			2.90%
Balance	$ 1,035,022,000	$ 1,049,959,000	$ 78,246,000	$ 78,246,000	$ 268,776,000	$ 269,496,000	$ 320,000,000		$ 300,000,000	$ 314,217,000	$ 320,000,000	$ 320,000,000		$ 68,000,000	$ 68,000,000
Basis spread over LIBOR (in percentage)					1.85%			0.95%
Mortgage Loan Converted To Recourse							75,000,000						75,000,000
Proceeds from (Repayments of) Debt								300,000,000
Number Of Extensions Available (Extensions)								4
Length Of Extension Available								1 year
Derivative Notional Amount								$ 300,000,000
Derivative cap interest rate, LIBOR								6.00%