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Schedule III: Rollforward of Real Estate Assets and Accumulated Depreciation (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Real Estate and Accumulated Depreciation [Line Items] |  |  |
Balance at beginning of period | $ 919,576 us-gaap_RealEstateGrossAtCarryingValue | $ 911,792 us-gaap_RealEstateGrossAtCarryingValue | $ 906,907 us-gaap_RealEstateGrossAtCarryingValue
Subtotal real estate | 993,984 alx_RealEstateAccumulatedDepreciationBeforeAccumulatedDepreciationOnAssetSoldAndWrittenOff | 919,576 alx_RealEstateAccumulatedDepreciationBeforeAccumulatedDepreciationOnAssetSoldAndWrittenOff | 911,792 alx_RealEstateAccumulatedDepreciationBeforeAccumulatedDepreciationOnAssetSoldAndWrittenOff
Less: Fully depreciated assets | (57)us-gaap_RealEstateOtherDeductions | 0 us-gaap_RealEstateOtherDeductions | 0 us-gaap_RealEstateOtherDeductions
Balance at end of the period | 993,927 us-gaap_RealEstateGrossAtCarryingValue | 919,576 us-gaap_RealEstateGrossAtCarryingValue | 911,792 us-gaap_RealEstateGrossAtCarryingValue
Reconciliation of Real Estate Accumulated Depreciation [Roll Forward] |  |  |
Balance at beginning of period | 185,375 us-gaap_RealEstateAccumulatedDepreciation | 160,826 us-gaap_RealEstateAccumulatedDepreciation | 136,460 us-gaap_RealEstateAccumulatedDepreciation
Real Estate Accumulated Depreciation, Depreciation Expense | 24,707 us-gaap_SECScheduleIIIRealEstateAccumulatedDepreciationDepreciationExpense | 24,549 us-gaap_SECScheduleIIIRealEstateAccumulatedDepreciationDepreciationExpense | 24,366 us-gaap_SECScheduleIIIRealEstateAccumulatedDepreciationDepreciationExpense
Subtotal of accumulated depreciation | 210,082 alx_RealEstateBeforeRealEstateSoldAndWrittenOff | 185,375 alx_RealEstateBeforeRealEstateSoldAndWrittenOff | 160,826 alx_RealEstateBeforeRealEstateSoldAndWrittenOff
Less: Fully depreciated assets | (57)us-gaap_RealEstateAccumulatedDepreciationRealEstateSold | 0 us-gaap_RealEstateAccumulatedDepreciationRealEstateSold | 0 us-gaap_RealEstateAccumulatedDepreciationRealEstateSold
Balance at end of the period | 210,025 us-gaap_RealEstateAccumulatedDepreciation | 185,375 us-gaap_RealEstateAccumulatedDepreciation | 160,826 us-gaap_RealEstateAccumulatedDepreciation
Land Improvements [Member] |  |  |
Real Estate and Accumulated Depreciation [Line Items] |  |  |
Real Estate, Improvements | 0 us-gaap_RealEstateImprovements / us-gaap_MajorPropertyClassAxis = us-gaap_LandImprovementsMember | 0 us-gaap_RealEstateImprovements / us-gaap_MajorPropertyClassAxis = us-gaap_LandImprovementsMember | 0 us-gaap_RealEstateImprovements / us-gaap_MajorPropertyClassAxis = us-gaap_LandImprovementsMember
Building Improvements [Member] |  |  |
Real Estate and Accumulated Depreciation [Line Items] |  |  |
Real Estate, Improvements | 4,043 us-gaap_RealEstateImprovements / us-gaap_MajorPropertyClassAxis = us-gaap_BuildingImprovementsMember | 5,072 us-gaap_RealEstateImprovements / us-gaap_MajorPropertyClassAxis = us-gaap_BuildingImprovementsMember | 3,776 us-gaap_RealEstateImprovements / us-gaap_MajorPropertyClassAxis = us-gaap_BuildingImprovementsMember
Construction in Progress [Member] |  |  |
Real Estate and Accumulated Depreciation [Line Items] |  |  |
Real Estate, Improvements | $ 70,365 us-gaap_RealEstateImprovements / us-gaap_MajorPropertyClassAxis = us-gaap_ConstructionInProgressMember | $ 2,712 us-gaap_RealEstateImprovements / us-gaap_MajorPropertyClassAxis = us-gaap_ConstructionInProgressMember | $ 1,109 us-gaap_RealEstateImprovements / us-gaap_MajorPropertyClassAxis = us-gaap_ConstructionInProgressMember

X
- Definition The subtotal of accumulated depreciation balance excluding accumulated depreciation on assets sold and write-off of accumulated depreciation on fully depreciated assets. + References No definition available. + Details
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- Definition The subtotal of real estate asset balance excluding assets sold and write-off of fully depreciated assets. + References No definition available. + Details
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- Definition Amount of accumulated depreciation pertaining to real estate investments for entities with a substantial portion of business acquiring and holding investment real estate. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 970 -SubTopic 360 -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-28.FN4) -URI http://asc.fasb.org/extlink&oid=6590653&loc=d3e638233-123024 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 10 -Section S99 -Paragraph 6 -Subparagraph (SX 210.5-04.(c) Schedule III) -URI http://asc.fasb.org/extlink&oid=27047687&loc=d3e5864-122674 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 28 -Article 12 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph c -Subparagraph Schedule III -Article 5 + Details
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- Definition Amount of decrease to accumulated depreciation for real estate investments sold for entities with a substantial portion of business acquiring and holding investment real estate. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 970 -SubTopic 360 -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-28.FN4) -URI http://asc.fasb.org/extlink&oid=6590653&loc=d3e638233-123024 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 10 -Section S99 -Paragraph 6 -Subparagraph (SX 210.5-04.(c) Schedule III) -URI http://asc.fasb.org/extlink&oid=27047687&loc=d3e5864-122674 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 28 -Article 12 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph c -Subparagraph Schedule III -Article 5 + Details
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- Definition Amount, before accumulated depreciation, of real estate investments for entities with a substantial portion of business acquiring and holding investment real estate. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 10 -Section S99 -Paragraph 6 -Subparagraph (SX 210.5-04.(c) Schedule III) -URI http://asc.fasb.org/extlink&oid=27047687&loc=d3e5864-122674 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 28 -Article 12 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph c -Subparagraph Schedule III -Article 5 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 970 -SubTopic 360 -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-28.4) -URI http://asc.fasb.org/extlink&oid=6590653&loc=d3e638233-123024 + Details
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- Definition Amount of increase in real estate investments related to improvements for entities with a substantial portion of business acquiring and holding investment real estate. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 10 -Section S99 -Paragraph 6 -Subparagraph (SX 210.5-04.(c) Schedule III) -URI http://asc.fasb.org/extlink&oid=27047687&loc=d3e5864-122674 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 28 -Article 12 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph c -Subparagraph Schedule III -Article 5 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 970 -SubTopic 360 -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-28.4) -URI http://asc.fasb.org/extlink&oid=6590653&loc=d3e638233-123024 + Details
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