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Marketable Securities (Details) (USD $) In Thousands, except Share data, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013
Investment Holdings [Line Items] |  |
Fair Value | $ 44,646 us-gaap_AvailableForSaleSecurities | $ 31,522 us-gaap_AvailableForSaleSecurities
Macerich interest [Member] |  |
Investment Holdings [Line Items] |  |
Macerich Common Shares | 535,265 us-gaap_InvestmentOwnedBalanceShares / invest_InvestmentIssuerAxis = alx_MacerichMember | 535,265 us-gaap_InvestmentOwnedBalanceShares / invest_InvestmentIssuerAxis = alx_MacerichMember
Economic basis per share (in dollars per share) | $ 56.05 alx_EconomicBasisOfMarketableSecuritiesPerShare / invest_InvestmentIssuerAxis = alx_MacerichMember |
GAAP Cost | 30,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / invest_InvestmentIssuerAxis = alx_MacerichMember |
Fair Value | 44,646 us-gaap_AvailableForSaleSecurities / invest_InvestmentIssuerAxis = alx_MacerichMember | 31,522 us-gaap_AvailableForSaleSecurities / invest_InvestmentIssuerAxis = alx_MacerichMember
Closing share price (in dollars per share) | $ 83.41 us-gaap_SharePrice / invest_InvestmentIssuerAxis = alx_MacerichMember | $ 58.89 us-gaap_SharePrice / invest_InvestmentIssuerAxis = alx_MacerichMember
Unrealized Gains | $ 13,124 us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLossNetOfTax / invest_InvestmentIssuerAxis = alx_MacerichMember | $ 316 us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLossNetOfTax / invest_InvestmentIssuerAxis = alx_MacerichMember

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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562 + Details
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