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Real Estate Investments (Details) - Acquired Intangibles (USD $)
Dec. 31, 2014
Dec. 31, 2013
Acquired Finite-Lived Intangible Assets [Line Items]    
Acquired Below-Market Lease Intangibles (weighted average life of 81 and 0 months, respectively) $ 74,191us-gaap_OffMarketLeaseUnfavorable $ 0us-gaap_OffMarketLeaseUnfavorable
Leases, Acquired-in-Place [Member]    
Acquired Finite-Lived Intangible Assets [Line Items]    
Acquired Intangible Lease Assets (in-place lease intangibles with a weighted average life of 140 and 0 months, respectively) 522,129us-gaap_FiniteLivedIntangibleAssetAcquiredInPlaceLeases
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_LeasesAcquiredInPlaceMember		 0us-gaap_FiniteLivedIntangibleAssetAcquiredInPlaceLeases
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_LeasesAcquiredInPlaceMember
Acquired Intangible Lease Assets (in-place lease intangibles with a weighted average life of 140 and 0 months, respectively) 24,233aei21_FiniteLivedIntangibleAssetsAccumulatedAmortizationInPlace
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_LeasesAcquiredInPlaceMember		 0aei21_FiniteLivedIntangibleAssetsAccumulatedAmortizationInPlace
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_LeasesAcquiredInPlaceMember
Off Market Unfavorable Lease    
Acquired Finite-Lived Intangible Assets [Line Items]    
Acquired Below-Market Lease Intangibles (weighted average life of 81 and 0 months, respectively) 80,603us-gaap_OffMarketLeaseUnfavorable
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= aei21_OffMarketUnfavorableLeaseMember		 0us-gaap_OffMarketLeaseUnfavorable
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= aei21_OffMarketUnfavorableLeaseMember
Acquired Below-Market Lease Intangibles (weighted average life of 81 and 0 months, respectively) $ 6,412aei21_FiniteLivedIntangibleAssetsAccumulatedAmortizationBelowMarket
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= aei21_OffMarketUnfavorableLeaseMember		 $ 0aei21_FiniteLivedIntangibleAssetsAccumulatedAmortizationBelowMarket
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= aei21_OffMarketUnfavorableLeaseMember