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Stock- Based Compensation (Details) (USD $) | 12 Months Ended | 1 Months Ended
--- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | May 31, 2014
Stock-Based Compensation [Abstract] |  |  |  |
Shares available for future grant under the plan | 889,735 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Expense recognized in connection with the issuance of Deferred Stock Units (in US dollars) | $ 394,000 us-gaap_RestrictedStockExpense | $ 394,000 us-gaap_RestrictedStockExpense | $ 300,000 us-gaap_RestrictedStockExpense |
Deferred Stock Units [Member] |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award Non Option Equity Instruments Granted Per Director (in shares) |  |  |  | 212 alx_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGrantedPerDirector / us-gaap_AwardTypeAxis = alx_DeferredStockUnitsMember
Expense recognized in connection with the issuance of Deferred Stock Units (in US dollars) |  |  |  | 394,000 us-gaap_RestrictedStockExpense / us-gaap_AwardTypeAxis = alx_DeferredStockUnitsMember
Expense per grant recognized in connection with the issuance of Deferred Stock Units (in US dollars) |  |  |  | $ 56,250 alx_RestrictedStockExpenseFairValuePerGrant / us-gaap_AwardTypeAxis = alx_DeferredStockUnitsMember
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Outstanding Number | 5,005 alx_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingNumber / us-gaap_AwardTypeAxis = alx_DeferredStockUnitsMember |  |  |

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- Definition The noncash expense that represents the cost of restricted stock or unit distributed to employees as compensation. Represents the expense recognized for each grantee. + References No definition available. + Details
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- Definition Outstanding number of share based awards other than options. + References No definition available. + Details
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- Definition Number of restricted stock units granted to each of the members of the board of directors during the period + References No definition available. + Details
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- Definition The noncash expense that represents the cost of restricted stock or unit distributed to employees as compensation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 + Details
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- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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