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Commitments and Contingencies (Details) (USD $) |  |  |  | 9 Months Ended |  | 12 Months Ended | 9 Months Ended
--- | --- | --- | --- | --- | --- | --- | ---
Sep. 30, 2013 | Dec. 31, 2012 | Sep. 30, 2013 Lexington Avenue Property [Member] | Sep. 30, 2013 Paramus Property [Member] Tenant Occupant [Member] Ikea [Member] | Dec. 31, 2001 Paramus Property [Member] Tenant Occupant [Member] Ikea [Member] acre | Dec. 31, 2021 Paramus Property [Member] Tenant Occupant [Member] Ikea [Member] Scenario, Forecast [Member] | Sep. 30, 2013 Flushing Property [Member] | Sep. 30, 2013 General Liability [Member] | Sep. 30, 2013 All Risk Property And Rental Value [Member] | Sep. 30, 2013 Terrorism Coverage Including Nbcr [Member] | Sep. 30, 2013 NBCR [Member]
Insurance [Abstract] |  |  |  |  |  |  |  |  |  |  |
Insurance Maximum Coverage Per Incident |  |  |  |  |  |  |  | $ 300,000,000 | $ 1,700,000,000 | $ 1,700,000,000 |
Deductible |  |  |  |  |  |  |  |  |  |  | 275,000
Self Insured Responsibility |  |  |  |  |  |  |  |  |  |  | 15.00%
Federal Government Responsibility |  |  |  |  |  |  |  |  |  |  | 85.00%
Mortgage Loan Converted To Recourse (in US Dollars) |  |  | 75,000,000 |  |  |  |  |  |  |  |
Mortgage payable (in US Dollars) | 1,054,046,000 | 1,065,916,000 | 320,000,000 | 68,000,000 |  |  |  |  |  |  |
Flushing Property [Abstract] |  |  |  |  |  |  |  |  |  |  |
Non-refundable purchase deposit |  |  |  |  |  |  | 1,875,000 |  |  |  |
Indemnification Sought by Tenant |  |  |  |  |  |  | 50,000,000 |  |  |  |
Paramus Property [Abstract] |  |  |  |  |  |  |  |  |  |  |
Acres Of Land |  |  |  |  | 30.3 |  |  |  |  |  |
Leases Of Lessor Rental Term Range |  |  |  | 40 years |  |  |  |  |  |  |
Property purchase option exercisable by lessee |  |  |  |  |  | 75,000,000 |  |  |  |  |
Fixed interest rate on the debt |  |  |  | 2.90% |  |  |  |  |  |  |
Triple-net rent, annual amount |  |  |  | 700,000 |  |  |  |  |  |  |
Purchase option exercised, proceeds from sale of land |  |  |  |  |  | 7,000,000 |  |  |  |  |
Purchase option exercised, gain on sale of land |  |  |  |  |  | 60,000,000 |  |  |  |  |
Purchase option not exercised, amount included in triple net rent over remainder of lease |  |  |  |  |  | 68,000,000 |  |  |  |  |
Loan Amortization Period |  |  |  |  |  | 20 years |  |  |  |  |
Letters Of Credit [Abstract] |  |  |  |  |  |  |  |  |  |  |
Standby letters of credit, issued and outstanding | $ 3,308,000 |  |  |  |  |  |  |  |  |  |

X
- Definition The deductible owed by our wholly owned insurance subsidiary in the event of a covered loss + References No definition available. + Details
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X
- Definition The percentage of the covered portion of a loss above the deductible for which the Federal Government is responsible for covered losses + References No definition available. + Details
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Balance Type: | na
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X
- Details
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X
- Definition Insurance maximum coverage limit per incident + References No definition available. + Details
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Balance Type: | debit
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X
- Definition Approximate rental term range for all outstanding leases to tenants in retail centers and office buildings. + References No definition available. + Details
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- Details
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- Definition In the event the tenant does not exercise its purchase option, the period over which the tenant's debt service portion of its rent must be sufficient to fully amortize the loan on the property + References No definition available. + Details
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X
- Definition Indemnification sought by the tenant for legal action brought against the tenant by a 3rd party + References No definition available. + Details
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- Definition Amount of loan that will become recourse in case of a substantial casualty + References No definition available. + Details
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- Details
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X
- Definition Lease with purchase option to be exercised by lessee + References No definition available. + Details
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Namespace Prefix: | alx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The cash inflow from the sale of land held for use in case the purchase option is exercised by lessee. + References No definition available. + Details
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Namespace Prefix: | alx_
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X
- Definition The amount to be amortized over the remainder of the lease term if the purchase option is not exercised + References No definition available. + Details
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X
- Definition The percentage of the covered portion of a loss above the deductible for which our wholly owned insurance subsidiary is responsible for covered losses + References No definition available. + Details
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Period Type: | duration

X
- Definition The fixed minimum triple-net annual rent + References No definition available. + Details
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X
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- 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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- Definition The difference between the carrying value and the sale price of real estate or properties that were intended to be sold or held for capital appreciation or rental income. This element refers to the gain (loss) included in earnings and not to the cash proceeds of the sale. This element is a noncash adjustment to net income when calculating net cash generated by operating activities using the indirect method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 28 -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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- Details
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- Definition The total amount of the contingent obligation under letters of credit outstanding as of the reporting date. + 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 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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- Definition This element represents money paid in advance to protect the provider of a product or service, such as a lessor, against damage or nonpayment by the buyer or tenant (lessee) during the term of the agreement. Such damages may include physical damage to the property, theft of property, and other contractual breaches. Security deposits held may be interest or noninterest bearing. + 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 210.7-03.15(a)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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