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Date when the acquirer obtains control of the acquiree, in YYYY-MM-DD format.

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-Name Accounting Standards Codification
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Data Type:	xbrli:dateItemType
Balance Type:	na
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The aggregate costs of keeping the property in good condition but that do not appreciably prolong the life or increase the value of the property.

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-Name Accounting Standards Codification
-Topic 220
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-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.2)
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Data Type:	xbrli:monetaryItemType
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Period Type:	duration

Date the entity disposed of a business or a group of assets, in YYYY-MM-DD format.

No definition available.

Name:	us-gaap_DisposalDate1
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This element represents the amount of value allocated by a lessor (acquirer) to lease agreements which exist at acquisition of a leased property. Such amount may include the value assigned to existing tenant relationships and excludes the market adjustment component of the value assigned for above or below-market leases acquired.

No definition available.

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Period Type:	instant

Amount of gain (loss) on sale or disposal of assets, including but not limited to property plant and equipment, intangible assets and equity in securities of subsidiaries or equity method investee.

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Amount of cash outflow for lease commissions incurred.

No definition available.

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Amount of cash outflow for the allowance granted to lessee and/or direct costs incurred by lessor used to prepare the leased premises for tenant's occupancy.

No definition available.

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The cash outflow from the acquisition of a piece of land, anything permanently fixed to it, including buildings, structures on it and so forth; includes real estate intended to generate income for the owner; excludes real estate acquired for use by the owner.

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Cash received for the sale of real estate that is not part of an investing activity during the current period.

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Amount of accumulated depreciation pertaining to real estate investments for entities with a substantial portion of business acquiring and holding investment real estate.

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Period Type:	instant

Amount of real estate investments sold for entities with a substantial portion of business acquiring and holding investment real estate.

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Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

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Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=aei21_FiveBelowChampaignILMember
Namespace Prefix:
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Namespace Prefix:
Data Type:	na
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Name:	us-gaap_BusinessAcquisitionAxis=aei21_AdvanceAutoChelseaALMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_LeasesAcquiredInPlaceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=aei21_BurlingtonCoatFactoryChampaignILMember
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Name:	srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=aei21_FiveBelowChampaignILMember
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Name:	srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=aei21_JaredJewelryAuburnHillsMIMember
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