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Income Taxes (Details) - Federal Tax Partners' Capital (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Federal Tax Partners' Capital [Abstract]    
Partners' Capital for Financial Reporting Purposes $ 15,591,281us-gaap_PartnersCapitalOther $ 15,889,559us-gaap_PartnersCapitalOther
Adjusted Tax Basis of Investments in Real Estate Over Net Investments in Real Estate for Financial Reporting Purposes 886,357aei21_AdjustedTaxBasisOfInvestmentsInRealEstateOverNetInvestmentsInRealEstateForFinanc 889,988aei21_AdjustedTaxBasisOfInvestmentsInRealEstateOverNetInvestmentsInRealEstateForFinanc
Income Accrued for Tax Purposes Over Income for Financial Reporting Purposes 12,121aei21_IncomeAccruedForTaxPurposesOverIncomeForFinancialReportingPurposes 12,121aei21_IncomeAccruedForTaxPurposesOverIncomeForFinancialReportingPurposes
Syndication Costs Treated as Reduction of Capital For Financial Reporting Purposes 3,208,043aei21_SyndicationCostsTreatedAsReductionOfCapitalForFinancialReportingPurposes 3,208,043aei21_SyndicationCostsTreatedAsReductionOfCapitalForFinancialReportingPurposes
Partners' Capital for Tax Reporting Purposes $ 19,697,802aei21_PartnersCapitalForTaxReportingPurposes $ 19,999,711aei21_PartnersCapitalForTaxReportingPurposes