Real Estate, Operating Real Estate, Real Estate Under Construction, and Equity Investment in Real Estate - Rollforward of Real Estate Under Construction (Details) $ in Thousands	9 Months Ended
Sep. 30, 2018 USD ($)
Real Estate Under Construction
Beginning balance	$ 134,366
Capitalized funds	133,325
Placed into service	(123,817)
Foreign currency translation adjustments	(4,596)
Capitalized interest	4,288
Ending balance	$ 143,566

Amount of assets under construction that are now placed into service as the balance sheet date.

No definition available.

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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- Definition

Other impacts on the carrying value of real estate under construction that includes but is not limited to foreign currency translation adjustments and building improvements.

+ References

No definition available.

+ Details

Name:	cpa18_ForeignCurrencyTranslationAndOtherImpactsonRealEstateUnderConstruction
Namespace Prefix:	cpa18_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Construction costs capitalized to date on capital projects that have not been completed and assets being constructed that are not ready to be placed into service.

+ References

No definition available.

+ Details

Name:	cpa18_FundsCapitalizedForConstructionInProgress
Namespace Prefix:	cpa18_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Real Estate Under Construction [Roll Forward]

+ References

No definition available.

+ Details

Name:	cpa18_RealEstateUnderConstructionRollForward
Namespace Prefix:	cpa18_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of structure or a modification to a structure under construction. Includes recently completed structures or modifications to structures that have not been placed into service.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229

+ Details

Name:	us-gaap_ConstructionInProgressGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of interest capitalized during the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6450988&loc=d3e26243-108391

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration