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Marketable Securities (Details) (USD $)
In Thousands, except Share data, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Investment Holdings [Line Items]    
Fair Value $ 45,139us-gaap_AvailableForSaleSecurities $ 44,646us-gaap_AvailableForSaleSecurities
Macerich interest [Member]    
Investment Holdings [Line Items]    
Macerich Common Shares 535,265us-gaap_InvestmentOwnedBalanceShares
/ invest_InvestmentIssuerAxis
= alx_MacerichMember		 535,265us-gaap_InvestmentOwnedBalanceShares
/ invest_InvestmentIssuerAxis
= alx_MacerichMember
Economic basis per share (in dollars per share) $ 56.05alx_EconomicBasisOfMarketableSecuritiesPerShare
/ invest_InvestmentIssuerAxis
= alx_MacerichMember		  
GAAP Cost 30,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentIssuerAxis
= alx_MacerichMember		  
Fair Value $ 45,139us-gaap_AvailableForSaleSecurities
/ invest_InvestmentIssuerAxis
= alx_MacerichMember		 $ 44,646us-gaap_AvailableForSaleSecurities
/ invest_InvestmentIssuerAxis
= alx_MacerichMember
Closing share price (in dollars per share) $ 84.33us-gaap_SharePrice
/ invest_InvestmentIssuerAxis
= alx_MacerichMember		 $ 83.41us-gaap_SharePrice
/ invest_InvestmentIssuerAxis
= alx_MacerichMember