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Major Tenants (Details) - Major Tenants (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Revenue, Major Customer [Line Items]    
Major Tenants $ 909,393us-gaap_Revenues $ 999,807us-gaap_Revenues
Aggregate rental income of major tenants as a percentage of total rental income 80.00%us-gaap_ConcentrationRiskPercentage1 79.00%us-gaap_ConcentrationRiskPercentage1
Sterling Jewelers Inc    
Revenue, Major Customer [Line Items]    
Major Tenants 298,178us-gaap_Revenues
/ us-gaap_MajorCustomersAxis
= aei21_SterlingJewelersIncMember
298,178us-gaap_Revenues
/ us-gaap_MajorCustomersAxis
= aei21_SterlingJewelersIncMember
Best Buy Stores LP    
Revenue, Major Customer [Line Items]    
Major Tenants 268,800us-gaap_Revenues
/ us-gaap_MajorCustomersAxis
= aei21_BestBuyStoresLPMember
268,146us-gaap_Revenues
/ us-gaap_MajorCustomersAxis
= aei21_BestBuyStoresLPMember
Tractor Supply Company    
Revenue, Major Customer [Line Items]    
Major Tenants 196,968us-gaap_Revenues
/ us-gaap_MajorCustomersAxis
= aei21_TractorSupplyCompanyMember
141,750us-gaap_Revenues
/ us-gaap_MajorCustomersAxis
= aei21_TractorSupplyCompanyMember
KinderCare Learning Centers LLC    
Revenue, Major Customer [Line Items]    
Major Tenants 145,447us-gaap_Revenues
/ us-gaap_MajorCustomersAxis
= aei21_KinderCareLearningCentersLLCMember
145,447us-gaap_Revenues
/ us-gaap_MajorCustomersAxis
= aei21_KinderCareLearningCentersLLCMember
CarMax Auto Superstores Inc    
Revenue, Major Customer [Line Items]    
Major Tenants    $ 146,286us-gaap_Revenues
/ us-gaap_MajorCustomersAxis
= aei21_CarMaxAutoSuperstoresIncMember