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Leases (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 12 Months Ended | 1 Months Ended
--- | --- | --- | ---
Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Oct. 31, 2014 sqft
Operating Leases, Future Minimum Payments Receivable [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
2015 | $ 127,877 us-gaap_OperatingLeasesFutureMinimumPaymentsReceivableCurrent |  |  |  |  |  |  |  | $ 127,877 us-gaap_OperatingLeasesFutureMinimumPaymentsReceivableCurrent |  |  |
2016 | 119,741 us-gaap_OperatingLeasesFutureMinimumPaymentsReceivableInTwoYears |  |  |  |  |  |  |  | 119,741 us-gaap_OperatingLeasesFutureMinimumPaymentsReceivableInTwoYears |  |  |
2017 | 121,079 us-gaap_OperatingLeasesFutureMinimumPaymentsReceivableInThreeYears |  |  |  |  |  |  |  | 121,079 us-gaap_OperatingLeasesFutureMinimumPaymentsReceivableInThreeYears |  |  |
2018 | 121,111 us-gaap_OperatingLeasesFutureMinimumPaymentsReceivableInFourYears |  |  |  |  |  |  |  | 121,111 us-gaap_OperatingLeasesFutureMinimumPaymentsReceivableInFourYears |  |  |
2019 | 118,537 us-gaap_OperatingLeasesFutureMinimumPaymentsReceivableInFiveYears |  |  |  |  |  |  |  | 118,537 us-gaap_OperatingLeasesFutureMinimumPaymentsReceivableInFiveYears |  |  |
Thereafter | 903,901 us-gaap_OperatingLeasesFutureMinimumPaymentsReceivableThereafter |  |  |  |  |  |  |  | 903,901 us-gaap_OperatingLeasesFutureMinimumPaymentsReceivableThereafter |  |  |
Additional Rent Based on Percentage of Tenant's sales |  |  |  |  |  |  |  |  | 108 us-gaap_PercentageRent | 416 us-gaap_PercentageRent | 416 us-gaap_PercentageRent |
Real Estate Properties [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
Real estate revenue, net (in dollars) | 51,286 us-gaap_RealEstateRevenueNet | 50,077 us-gaap_RealEstateRevenueNet | 49,983 us-gaap_RealEstateRevenueNet | 49,468 us-gaap_RealEstateRevenueNet | 50,496 us-gaap_RealEstateRevenueNet | 49,886 us-gaap_RealEstateRevenueNet | 47,302 us-gaap_RealEstateRevenueNet | 48,775 us-gaap_RealEstateRevenueNet | 200,814 us-gaap_RealEstateRevenueNet | 196,459 us-gaap_RealEstateRevenueNet | 191,312 us-gaap_RealEstateRevenueNet |
Operating Leases, Future Minimum Payments Due [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
2015 | 700 us-gaap_OperatingLeasesFutureMinimumPaymentsDueCurrent |  |  |  |  |  |  |  | 700 us-gaap_OperatingLeasesFutureMinimumPaymentsDueCurrent |  |  |
2016 | 700 us-gaap_OperatingLeasesFutureMinimumPaymentsDueInTwoYears |  |  |  |  |  |  |  | 700 us-gaap_OperatingLeasesFutureMinimumPaymentsDueInTwoYears |  |  |
2017 | 792 us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears |  |  |  |  |  |  |  | 792 us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears |  |  |
2018 | 800 us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears |  |  |  |  |  |  |  | 800 us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears |  |  |
2019 | 800 us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFiveYears |  |  |  |  |  |  |  | 800 us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFiveYears |  |  |
Thereafter | 5,666 us-gaap_OperatingLeasesFutureMinimumPaymentsDueThereafter |  |  |  |  |  |  |  | 5,666 us-gaap_OperatingLeasesFutureMinimumPaymentsDueThereafter |  |  |
Rent expense |  |  |  |  |  |  |  |  | 746 us-gaap_LeaseAndRentalExpense | 746 us-gaap_LeaseAndRentalExpense | 746 us-gaap_LeaseAndRentalExpense |
Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |
Leases Lease Terms [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
Lease term range as Lessor |  |  |  |  |  |  |  |  | 5 years |  |  |
Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |
Leases Lease Terms [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
Lease term range as Lessor |  |  |  |  |  |  |  |  | 25 years |  |  |
Customer Concentration Risk [Member] | Bloomberg [Member] |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
Real estate revenue, net (in dollars) |  |  |  |  |  |  |  |  | $ 91,109 us-gaap_RealEstateRevenueNet / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = alx_BloombergMember | $ 88,164 us-gaap_RealEstateRevenueNet / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = alx_BloombergMember | $ 86,468 us-gaap_RealEstateRevenueNet / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = alx_BloombergMember |
Percentage Of Minimum Revenue Threshold Contributed By One Tenant (in percentage) |  |  |  |  |  |  |  |  | 10.00%alx_PercentageOfMinimumRevenueThresholdContributedByOneTenant / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = alx_BloombergMember |  |  |
Customer Concentration Risk [Member] | Bloomberg [Member] | Sales Revenue Services Net [Member] |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
Percentage Rent Contributed By Tenant, Minimum threshold |  |  |  |  |  |  |  |  | 45.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_SalesRevenueServicesNetMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = alx_BloombergMember | 45.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_SalesRevenueServicesNetMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = alx_BloombergMember | 45.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_SalesRevenueServicesNetMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CustomerConcentrationRiskMember / us-gaap_MajorCustomersAxis = alx_BloombergMember |
Flushing Property [Member] |  |  |  |  |  |  |  |  |  |  |  |
Lessee [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
Expiration year |  |  |  |  |  |  |  |  | 2027 |  |  |
Flushing Property [Member] | Ten Year Extension Option [Member] |  |  |  |  |  |  |  |  |  |  |  |
Lessee [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
Number Of Extension Options | 1 alx_NumberOfExtensionOptions / alx_ExtensionOptionTermsAxis = alx_TenYearExtensionOptionMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = alx_FlushingPropertyMember |  |  |  |  |  |  |  | 1 alx_NumberOfExtensionOptions / alx_ExtensionOptionTermsAxis = alx_TenYearExtensionOptionMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = alx_FlushingPropertyMember |  |  |
Lease Extension Period Maximum |  |  |  |  |  |  |  |  | 10 years |  |  |
731 Lexington Avenue [Member] | Bloomberg [Member] |  |  |  |  |  |  |  |  |  |  |  |
Real Estate Properties [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
Area of property (in square feet) |  |  |  |  |  |  |  |  |  |  |  | 188,608 us-gaap_AreaOfRealEstateProperty / us-gaap_MajorCustomersAxis = alx_BloombergMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = alx_LexingtonAvenuePropertyMember
Lease renewal term |  |  |  |  |  |  |  |  |  |  |  | 5 years

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- Definition Year of ground lease expiration + References No definition available. + Details
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- Definition This element represents number of years available to extend the lease on optional basis. + References No definition available. + Details
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- 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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- Definition The remaining number of extension options under a ground lease + References No definition available. + Details
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- Definition Percentage Of Minimum Revenue Threshold By One Tenant. + References No definition available. + Details
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- Definition Area of a real estate property. + References No definition available. + Details
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- Definition For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 21 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13537-108611 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6404-108592 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13531-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 18 -URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6351-108592 + Details
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- Definition Amount of rent expense incurred for leased assets, including but not limited to, furniture and equipment, that is not directly or indirectly associated with the manufacture, sale or creation of a product or product line. + References No definition available. + Details
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- Definition Term of the lessee's leasing arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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- Definition Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41502-112717 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 10 -Section 55 -Paragraph 40 -Subparagraph (Note 3) -URI http://asc.fasb.org/extlink&oid=6584154&loc=d3e38371-112697 + Details
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- Definition Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41502-112717 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 10 -Section 55 -Paragraph 40 -Subparagraph (Note 3) -URI http://asc.fasb.org/extlink&oid=6584154&loc=d3e38371-112697 + Details
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- Definition Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41502-112717 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 10 -Section 55 -Paragraph 40 -Subparagraph (Note 3) -URI http://asc.fasb.org/extlink&oid=6584154&loc=d3e38371-112697 + Details
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- Definition Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41502-112717 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 10 -Section 55 -Paragraph 40 -Subparagraph (Note 3) -URI http://asc.fasb.org/extlink&oid=6584154&loc=d3e38371-112697 + Details
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- Definition Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41502-112717 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 10 -Section 55 -Paragraph 40 -Subparagraph (Note 3) -URI http://asc.fasb.org/extlink&oid=6584154&loc=d3e38371-112697 + Details
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- Definition Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41502-112717 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 10 -Section 55 -Paragraph 40 -Subparagraph (Note 3) -URI http://asc.fasb.org/extlink&oid=6584154&loc=d3e38371-112697 + Details
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- Definition Future rental payments receivable within one year of the balance sheet date under an operating lease. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6454179&loc=d3e41551-112718 + Details
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- Definition Future rental payments receivable within the fifth year from the balance sheet date under an operating lease. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6454179&loc=d3e41551-112718 + Details
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- Definition Future rental payments receivable within the fourth year from the balance sheet date under an operating lease. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6454179&loc=d3e41551-112718 + Details
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- Definition Future rental payments receivable within the third year from the balance sheet date under an operating lease. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6454179&loc=d3e41551-112718 + Details
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- Definition Future rental payments receivable within the second year from the balance sheet date under an operating lease. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6454179&loc=d3e41551-112718 + Details
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- Definition Future minimum lease payments receivable under operating leases for periods greater than five years following the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6454179&loc=d3e41551-112718 + Details
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- Definition This element represents the rental revenue earned during the period from lessee-operators based on revenues generated in their operations, generally in excess of a base amount. Such rental revenue is generally stipulated in the lease agreement, usually will provide for a fixed percentage of revenue to be paid as additional (or possibly only) rent due the lessor, and may be based on gross revenues, net revenues, or multiple variations thereof. Percentage rent is often required under leases with retail outlets located on premises owned by hoteliers, cruise lines, others in the hospitality industry, and shopping mall operators, among others. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.1(e)) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 + Details
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- Definition The aggregate revenue from real estate operations during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.1(e)) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Article 5 + Details
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