Debt Instrument, Extended Term

No definition available.

Name:	gcnl_DebtInstrumentExtendedTerm
Namespace Prefix:	gcnl_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Line of Credit Facility, Accordion Feature, Higher Borrowing Capacity Option

+ References

No definition available.

+ Details

Name:	gcnl_LineofCreditFacilityAccordionFeatureHigherBorrowingCapacityOption
Namespace Prefix:	gcnl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line of Credit Facility, Accordion Feature, Increase Limit

+ References

No definition available.

+ Details

Name:	gcnl_LineofCreditFacilityAccordionFeatureIncreaseLimit
Namespace Prefix:	gcnl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line of Credit Facility, Borrowing Capacity, Minimum Increment

+ References

No definition available.

+ Details

Name:	gcnl_LineofCreditFacilityBorrowingCapacityMinimumIncrement
Namespace Prefix:	gcnl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Property Interest Acquired

+ References

No definition available.

+ Details

Name:	gcnl_PropertyInterestAcquired
Namespace Prefix:	gcnl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

+ Details

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=gcnl_HealthSpringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=gcnl_HealthSpringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_MortgagesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=gcnl_AceHardwareMortgageLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_LineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=gcnl_UnsecuredCreditFacilityJuly2015Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_FederalFundsEffectiveSwapRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_UnsecuredDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=gcnl_IncreaseAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=gcnl_UnsecuredTermLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementScenarioAxis=us-gaap_ScenarioForecastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Debt - Narrative (Details) - USD ($)	Apr. 01, 2017	Apr. 27, 2016	Jul. 20, 2015	Sep. 30, 2016	Mar. 29, 2016
HealthSpring mortgage loan
Debt Instrument [Line Items]
Property interest acquired (percent)		90.00%
HealthSpring mortgage loan \| Mortgages
Debt Instrument [Line Items]
Face amount of debt assumed				$ 22,400,000
Contractual stated interest rate (percent)				4.18%
Ace Hardware mortgage loan \| Mortgages
Debt Instrument [Line Items]
Contractual stated interest rate (percent)				5.588%
Line of Credit [Member] \| Unsecured Credit Facility (July 2015) [Member]
Debt Instrument [Line Items]
Credit facility borrowing capacity			$ 1,140,000,000.00
Line of Credit [Member] \| Unsecured Credit Facility (July 2015) [Member] \| Ten Year Swap Rate
Debt Instrument [Line Items]
Spread over variable rate (percent)			0.50%
Revolving Credit Facility \| Unsecured Credit Facility (July 2015) [Member]
Debt Instrument [Line Items]
Credit facility borrowing capacity			$ 500,000,000
Maximum increment to borrowing capacity available under accordion feature			860,000,000
Minimum increments allowed under accordion feature			50,000,000
Maximum borrowing capacity available under accordion feature			$ 2,000,000,000
Initial debt instrument term			4 years
Debt instrument extended term			1 year
Unsecured Debt [Member] \| Unsecured Credit Facility (July 2015) [Member]
Debt Instrument [Line Items]
Credit facility borrowing capacity			$ 640,000,000
Initial debt instrument term			4 years
Debt instrument extended term			1 year
Unsecured Debt [Member] \| Increase Agreement [Member]
Debt Instrument [Line Items]
Credit facility borrowing capacity					$ 715,000,000.0
Unsecured Term Loan [Member] \| Unsecured Credit Facility (July 2015) [Member]
Debt Instrument [Line Items]
Initial debt instrument term			5 years
Expected Reset \| Ace Hardware mortgage loan \| Mortgages
Debt Instrument [Line Items]
Contractual stated interest rate (percent)	8.90%
Expected Reset \| Ace Hardware mortgage loan \| Ten Year Swap Rate \| Mortgages
Debt Instrument [Line Items]
Spread over variable rate (percent)	5.00%