Name:	aei21_DistributionsPerUnitOfReturnOfCapital
Namespace Prefix:	aei21_
Data Type:	us-types:perUnitItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

Name:	aei21_PartnersCapitalDetailsLineItems
Namespace Prefix:	aei21_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Distribution of sales proceeds declared or paid to a common shareholder or unit-holder by LLC or LP.

No definition available.

Name:	aei21_SaleProceedsDistributionMadeToMemberOrLimitedPartner
Namespace Prefix:	aei21_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- References

No definition available.

Name:	aei21_SaleProceedsDistributionMadetoLimitedPartnerPerUnit
Namespace Prefix:	aei21_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Cash dividends declared or paid to each outstanding limited partnership unit during the reporting period.

Name:	us-gaap_DistributionsPerLimitedPartnershipUnitOutstanding
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

Total distributions to each class of partners (i.e., general, limited and preferred partners).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 5
-Subparagraph (SAB Topic 4.F)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187171-122770

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Name:	us-gaap_PartnersCapitalAccountDistributions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Total change in each class of partners' capital accounts during the year due to redemptions and adjustments to redemption value. All partners include general, limited and preferred partners.

Name:	us-gaap_PartnersCapitalAccountRedemptions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

The number of units redeemed during the year of each class of partners' capital account. Units represent shares of ownership of the general, limited, and preferred partners.

Name:	us-gaap_PartnersCapitalAccountUnitsRedeemed
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

Weighted average number of limited partnership units outstanding determined by relating the portion of time within a reporting period that limited partnership units have been outstanding to the total time in that period.