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Partners' Capital (Details) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Partners' Capital (Details) [Line Items]    
Partners' Capital Account, Distributions $ 1,167,687us-gaap_PartnersCapitalAccountDistributions $ 1,474,752us-gaap_PartnersCapitalAccountDistributions
Sale Proceeds Distribution Made To Member Or Limited Partner 262,444aei21_SaleProceedsDistributionMadeToMemberOrLimitedPartner 455,194aei21_SaleProceedsDistributionMadeToMemberOrLimitedPartner
Partners' Capital Account, Redemptions 664,178us-gaap_PartnersCapitalAccountRedemptions  
Limited Partner [Member]    
Partners' Capital (Details) [Line Items]    
Partners' Capital Account, Distributions 1,156,010us-gaap_PartnersCapitalAccountDistributions
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_LimitedPartnerMember
1,460,005us-gaap_PartnersCapitalAccountDistributions
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_LimitedPartnerMember
Distributions Per Limited Partnership Unit Outstanding, Basic (in Dollars per share) $ 51.50us-gaap_DistributionsPerLimitedPartnershipUnitOutstanding
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_LimitedPartnerMember
$ 64.45us-gaap_DistributionsPerLimitedPartnershipUnitOutstanding
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_LimitedPartnerMember
Weighted Average Limited Partnership Units Outstanding, Basic (in Shares) 22,447us-gaap_WeightedAverageLimitedPartnershipUnitsOutstanding
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_LimitedPartnerMember
22,653us-gaap_WeightedAverageLimitedPartnershipUnitsOutstanding
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_LimitedPartnerMember
DistributionsPerUnitOfNetIncome (in Dollars per Item) 51.50aei21_DistributionsPerUnitOfNetIncome
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_LimitedPartnerMember
64.45aei21_DistributionsPerUnitOfNetIncome
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_LimitedPartnerMember
Sale Proceeds Distribution Made To Member Or Limited Partner 259,820aei21_SaleProceedsDistributionMadeToMemberOrLimitedPartner
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_LimitedPartnerMember
450,642aei21_SaleProceedsDistributionMadeToMemberOrLimitedPartner
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_LimitedPartnerMember
SaleProceedsDistributionMadetoLimitedPartnerPerUnit 11.57aei21_SaleProceedsDistributionMadetoLimitedPartnerPerUnit
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_LimitedPartnerMember
19.88aei21_SaleProceedsDistributionMadetoLimitedPartnerPerUnit
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_LimitedPartnerMember
Partners' Capital Account, Units, Redeemed (in Shares) 824.40us-gaap_PartnersCapitalAccountUnitsRedeemed
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_LimitedPartnerMember
 
Partners' Capital Account, Redemptions 657,537us-gaap_PartnersCapitalAccountRedemptions
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_LimitedPartnerMember
 
General Partner [Member]    
Partners' Capital (Details) [Line Items]    
Partners' Capital Account, Distributions 11,677us-gaap_PartnersCapitalAccountDistributions
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_GeneralPartnerMember
14,747us-gaap_PartnersCapitalAccountDistributions
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_GeneralPartnerMember
Sale Proceeds Distribution Made To Member Or Limited Partner 2,624aei21_SaleProceedsDistributionMadeToMemberOrLimitedPartner
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_GeneralPartnerMember
4,552aei21_SaleProceedsDistributionMadeToMemberOrLimitedPartner
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_GeneralPartnerMember
Partners' Capital Account, Redemptions 6,641us-gaap_PartnersCapitalAccountRedemptions
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_GeneralPartnerMember
 
GeneralPartnerDistributionsFromRedemptions $ 6,641aei21_GeneralPartnerDistributionsFromRedemptions
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_GeneralPartnerMember