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

No definition available.

Name:	cpa18_CapitalInterestInOperatingPartnership
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

No definition available.

+ 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=109223946&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=109223946&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_ThirteenStudentHousingDevelopmentsMember
Namespace Prefix:
Data Type:	na
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- Details

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Namespace Prefix:
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Organization - Narratives (Details) ft² in Millions, $ in Millions	3 Months Ended	39 Months Ended	87 Months Ended
	Mar. 31, 2019 ft² segment property tenant	Apr. 02, 2015 USD ($)	Mar. 31, 2019 USD ($) ft² property tenant	Mar. 31, 2018 property
Public Offering
Capital interest ownership in operating partnership	99.97%		99.97%
Additional Disclosures
Number of properties	56		56
Number of tenants \| tenant	90		90
Area of real estate property (sqft) \| ft²	10.0		10.0
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 \| $			$ 158.8
Class C common stock
Additional Disclosures
Proceeds from dividend reinvestment plan \| $			$ 44.8
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	69		69	69
Operating Real Estate \| Multi-Family
Additional Disclosures
Number of properties	15		15
Operating Real Estate \| Multi-Family \| Thirteen Student Housing Developments
Additional Disclosures
Number of properties	13		13
Operating Real Estate \| Multi-Family \| Two Student Housing Operating Properties
Additional Disclosures
Number of properties	2		2