The percentage of capital interest in the Operating Partnership by the general partner.

No definition available.

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Namespace Prefix:	cpa18_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of tenants occupying the real estate properties owned as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	cpa18_NumberOfTenants
Namespace Prefix:	cpa18_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Offering [Abstract]

+ References

No definition available.

+ Details

Name:	cpa18_OfferingAbstract
Namespace Prefix:	cpa18_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Other Business Disclosures

+ References

No definition available.

+ Details

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Namespace Prefix:	cpa18_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Area of a real estate property.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:areaItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of real estate properties owned as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	us-gaap_NumberOfRealEstateProperties
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8672-108599

+ Details

Name:	us-gaap_NumberOfReportableSegments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The cash inflow associated with the amount received from entity's first offering of stock to the public.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3255-108585

+ Details

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

- Definition

Amount of cash inflow from capital contributions to an entity associated with a dividend reinvestment plan.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3255-108585

+ Details

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

- Details

Name:	us-gaap_StatementClassOfStockAxis=us-gaap_CommonClassAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementClassOfStockAxis=us-gaap_CommonClassCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RealEstatePropertiesAxis=cpa18_OperatingRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis=cpa18_SelfStorageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis=srt_MultifamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=cpa18_TwelveStudentHousingDevelopmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	cpa18_AgreementTypeAxis=cpa18_NetLeaseAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=cpa18_ThreeStudentHousingOperatingPropertiesMember
Namespace Prefix:
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Organization - Narratives (Details) ft² in Millions, $ in Millions	6 Months Ended	39 Months Ended	102 Months Ended
	Jun. 30, 2020 ft² segment property tenant	Apr. 02, 2015 USD ($)	Jun. 30, 2020 USD ($) ft² property tenant
Public Offering
Capital interest ownership in operating partnership (percent)	99.97%		99.97%
Additional Disclosures
Number of properties	47		47
Number of tenants \| tenant	65		65
Area of real estate property (sqft) \| ft²	9.6		9.6
Number of reportable segments \| segment	3
Proceeds from issuance of shares \| $		$ 1,200.0
Class A common stock
Additional Disclosures
Proceeds from dividend reinvestment plan \| $			$ 200.7
Class C common stock
Additional Disclosures
Proceeds from dividend reinvestment plan \| $			$ 57.6
Operating Real Estate
Additional Disclosures
Area of real estate property (sqft) \| ft²	5.5		5.5
Operating Real Estate \| Self Storage
Additional Disclosures
Number of properties	68		68
Operating Real Estate \| Other Operating Properties \| Twelve Student Housing Developments
Additional Disclosures
Number of properties	12		12
Operating Real Estate \| Other Operating Properties \| Twelve Student Housing Developments \| Net lease agreement
Additional Disclosures
Number of properties	10		10
Operating Real Estate \| Other Operating Properties \| Three Student Housing Operating Properties
Additional Disclosures
Number of properties	2		2