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, 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, Covenant, Aggregate Maximum Unhedged Variable Rate Debt, Percent of Asset Value

+ References

No definition available.

+ Details

Name:	gcnl_LineofCreditFacilityCovenantAggregateMaximumUnhedgedVariableRateDebtPercentofAssetValue
Namespace Prefix:	gcnl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line of Credit Facility, Covenant, Maximum Consolidated Leverage Ratio

+ References

No definition available.

+ Details

Name:	gcnl_LineofCreditFacilityCovenantMaximumConsolidatedLeverageRatio
Namespace Prefix:	gcnl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line of Credit Facility, Covenant, Maximum Consolidated Leverage Ratio After Material Acquisition

+ References

No definition available.

+ Details

Name:	gcnl_LineofCreditFacilityCovenantMaximumConsolidatedLeverageRatioAfterMaterialAcquisition
Namespace Prefix:	gcnl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line of Credit Facility, Covenant, Maximum Consolidated Leverage Ratio, Number of Quarters of Higher Maximum

+ References

No definition available.

+ Details

Name:	gcnl_LineofCreditFacilityCovenantMaximumConsolidatedLeverageRatioNumberofQuartersofHigherMaximum
Namespace Prefix:	gcnl_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Line of Credit Facility, Covenant, Maximum Total Secured Debt Ratio

+ References

No definition available.

+ Details

Name:	gcnl_LineofCreditFacilityCovenantMaximumTotalSecuredDebtRatio
Namespace Prefix:	gcnl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line of Credit Facility, Covenant, Maximum Total Secured Debt Ratio, Increase

+ References

No definition available.

+ Details

Name:	gcnl_LineofCreditFacilityCovenantMaximumTotalSecuredDebtRatioIncrease
Namespace Prefix:	gcnl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line of Credit Facility, Covenant, Maximum Total Secured Debt Ratio, Periods of Increased Percentage

+ References

No definition available.

+ Details

Name:	gcnl_LineofCreditFacilityCovenantMaximumTotalSecuredDebtRatioPeriodsofIncreasedPercentage
Namespace Prefix:	gcnl_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Line of Credit Facility, Covenant, Maximum Total Secured Recourse Debt Ratio

+ References

No definition available.

+ Details

Name:	gcnl_LineofCreditFacilityCovenantMaximumTotalSecuredRecourseDebtRatio
Namespace Prefix:	gcnl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line of Credit Facility, Covenant, Minimum Consolidated Fixed Charge Coverage Ratio

+ References

No definition available.

+ Details

Name:	gcnl_LineofCreditFacilityCovenantMinimumConsolidatedFixedChargeCoverageRatio
Namespace Prefix:	gcnl_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Line of Credit Facility, Covenant, Minimum Unsecured Interest Coverage Ratio

+ References

No definition available.

+ Details

Name:	gcnl_LineofCreditFacilityCovenantMinimumUnsecuredInterestCoverageRatio
Namespace Prefix:	gcnl_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Line of Credit Facility, Covenant, Tangible Net Worth

+ References

No definition available.

+ Details

Name:	gcnl_LineofCreditFacilityCovenantTangibleNetWorth
Namespace Prefix:	gcnl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line of Credit Facility, Covenant, Tangible Net Worth, Additional Percentage of Net Future Equity issuances

+ References

No definition available.

+ Details

Name:	gcnl_LineofCreditFacilityCovenantTangibleNetWorthAdditionalPercentageofNetFutureEquityissuances
Namespace Prefix:	gcnl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line of Credit Facility, Covenant, Tangible Net Worth, Reducing Percentage for Redemption Payments

+ References

No definition available.

+ Details

Name:	gcnl_LineofCreditFacilityCovenantTangibleNetWorthReducingPercentageforRedemptionPayments
Namespace Prefix:	gcnl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line of Credit Facility, Covenant, Tangible Net Worth, Value

+ References

No definition available.

+ Details

Name:	gcnl_LineofCreditFacilityCovenantTangibleNetWorthValue
Namespace Prefix:	gcnl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line of Credit facility, Covenant, Maximum Pool Value Contributed by Pool Properties Subject to Ground Leases

+ References

No definition available.

+ Details

Name:	gcnl_LineofCreditfacilityCovenantMaximumPoolValueContributedbyPoolPropertiesSubjecttoGroundLeases
Namespace Prefix:	gcnl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line of Credit facility, Covenant, Maximum Pool Value Contributed by Poop Properties Under Development or Assets Under Renovation

+ References

No definition available.

+ Details

Name:	gcnl_LineofCreditfacilityCovenantMaximumPoolValueContributedbyPoopPropertiesUnderDevelopmentorAssetsUnderRenovation
Namespace Prefix:	gcnl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line of Credit facility, Covenant, Maximum Pool Value Contributed by Single Property or Tenant

+ References

No definition available.

+ Details

Name:	gcnl_LineofCreditfacilityCovenantMaximumPoolValueContributedbySinglePropertyorTenant
Namespace Prefix:	gcnl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line of Credit facility, Covenant, Maximum Unsecured Leverage Ratio

+ References

No definition available.

+ Details

Name:	gcnl_LineofCreditfacilityCovenantMaximumUnsecuredLeverageRatio
Namespace Prefix:	gcnl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line of Credit facility, Covenant, Minimum Aggregate Leasing Percentage

+ References

No definition available.

+ Details

Name:	gcnl_LineofCreditfacilityCovenantMinimumAggregateLeasingPercentage
Namespace Prefix:	gcnl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line of Credit facility, Covenant, Minimum Pool Properties

+ References

No definition available.

+ Details

Name:	gcnl_LineofCreditfacilityCovenantMinimumPoolProperties
Namespace Prefix:	gcnl_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Line of Credit facility, Covenant, Minimum Unsecured Coverage Ratio

+ References

No definition available.

+ Details

Name:	gcnl_LineofCreditfacilityCovenantMinimumUnsecuredCoverageRatio
Namespace Prefix:	gcnl_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- 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

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://fasb.org/us-gaap/role/ref/legacyRef
-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=120391452&loc=d3e13212-122682

+ Details

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

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=gcnl_A2025TermLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_ExternalCreditRatingByGroupingAxis=us-gaap_ExternalCreditRatingInvestmentGradeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=gcnl_A2024TermLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=gcnl_A2023TermLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=gcnl_A2026TermLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=us-gaap_LineOfCreditMember
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_DebtInstrumentAxis=gcnl_UnsecuredRevolverCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

Debt - Narrative (Details)		3 Months Ended
Debt - Narrative (Details)	Dec. 18, 2020 USD ($)	Mar. 31, 2021 USD ($) realEstateProperty quarter	Mar. 31, 2021 USD ($) realEstateProperty quarter
2025 Term Loan \| Minimum \| LIBOR \| External Credit Rating, Investment Grade
Debt Instrument [Line Items]
Spread on LIBOR (percent)		0.90%
2025 Term Loan \| Maximum \| LIBOR \| External Credit Rating, Investment Grade
Debt Instrument [Line Items]
Spread on LIBOR (percent)		1.75%
2024 Term Loan \| Minimum \| LIBOR \| External Credit Rating, Investment Grade
Debt Instrument [Line Items]
Spread on LIBOR (percent)		0.90%
2024 Term Loan \| Maximum \| LIBOR \| External Credit Rating, Investment Grade
Debt Instrument [Line Items]
Spread on LIBOR (percent)		1.75%
2023 Term Loan \| Minimum \| LIBOR \| External Credit Rating, Investment Grade
Debt Instrument [Line Items]
Spread on LIBOR (percent)		0.90%
2023 Term Loan \| Maximum \| LIBOR \| External Credit Rating, Investment Grade
Debt Instrument [Line Items]
Spread on LIBOR (percent)		1.75%
Term Loans \| Keybank Loans
Debt Instrument [Line Items]
Compliance requirement - maximum consolidated leverage ratio (percent)		60.00%	60.00%
Compliance requirement - maximum consolidated leverage ratio after material acquisition (percent)		65.00%	65.00%
Compliance requirement - consecutive quarters after material acquisition subject to higher consolidated leverage ratio \| quarter			4
Compliance requirement - minimum consolidated tangible net worth (percent)		75.00%	75.00%
Compliance requirement - minimum consolidated tangible net worth		$ 2,000,000,000.0	$ 2,000,000,000.0
Compliance requirement - additional net future equity issuances (percent)		75.00%	75.00%
Compliance requirement - reduction for amount of payments used to redeem stock (percent)		75.00%	75.00%
Compliance requirement - minimum consolidated fixed charge coverage ratio		1.50	1.50
Compliance requirement - maximum total secured debt ratio (percent)		40.00%	40.00%
Compliance requirement - increase in maximum total secured coverage ratio after material acquisition (percent)		5.00%	5.00%
Compliance requirement - number of quarters subject to higher maximum total secured coverage ratio after material acquisition \| quarter		4	4
Compliance requirement - minimum unsecured interest coverage ratio		2.00	2.00
Compliance requirement - maximum total secured recourse debt ratio (percent)		10.00%	10.00%
Compliance requirement - maximum aggregate maximum unhedged variable rate debt (percent)		30.00%	30.00%
Term Loans \| 2025 Term Loan
Debt Instrument [Line Items]
Credit facility	$ 400,000,000
Term of debt instrument	5 years	5 years
Term Loans \| 2025 Term Loan \| External Credit Rating, Investment Grade
Debt Instrument [Line Items]
Term of debt instrument		5 years
Term Loans \| 2025 Term Loan \| LIBOR
Debt Instrument [Line Items]
Spread on LIBOR (percent)			1.55%
Term Loans \| 2025 Term Loan \| Minimum \| LIBOR
Debt Instrument [Line Items]
Spread on LIBOR (percent)		1.25%
Term Loans \| 2025 Term Loan \| Maximum \| LIBOR
Debt Instrument [Line Items]
Spread on LIBOR (percent)		2.15%
Term Loans \| 2024 Term Loan
Debt Instrument [Line Items]
Credit facility	$ 400,000,000
Term of debt instrument	5 years	5 years
Term Loans \| 2024 Term Loan \| LIBOR
Debt Instrument [Line Items]
Spread on LIBOR (percent)			1.55%
Term Loans \| 2024 Term Loan \| Minimum \| LIBOR
Debt Instrument [Line Items]
Spread on LIBOR (percent)		1.25%
Term Loans \| 2024 Term Loan \| Maximum \| LIBOR
Debt Instrument [Line Items]
Spread on LIBOR (percent)		2.15%
Term Loans \| 2023 Term Loan
Debt Instrument [Line Items]
Credit facility	$ 200,000,000
Term of debt instrument	5 years	5 years
Term Loans \| 2023 Term Loan \| External Credit Rating, Investment Grade
Debt Instrument [Line Items]
Term of debt instrument		5 years
Term Loans \| 2023 Term Loan \| LIBOR
Debt Instrument [Line Items]
Spread on LIBOR (percent)			1.55%
Term Loans \| 2023 Term Loan \| Minimum \| LIBOR
Debt Instrument [Line Items]
Spread on LIBOR (percent)		1.25%
Term Loans \| 2023 Term Loan \| Maximum \| LIBOR
Debt Instrument [Line Items]
Spread on LIBOR (percent)		2.15%
Term Loans \| 2026 Term Loan
Debt Instrument [Line Items]
Credit facility	$ 150,000,000
Term of debt instrument		7 years
Term Loans \| 2026 Term Loan \| LIBOR
Debt Instrument [Line Items]
Spread on LIBOR (percent)			1.85%
Term Loans \| 2026 Term Loan \| Minimum \| LIBOR
Debt Instrument [Line Items]
Spread on LIBOR (percent)		1.65%
Term Loans \| 2026 Term Loan \| Minimum \| LIBOR \| External Credit Rating, Investment Grade
Debt Instrument [Line Items]
Spread on LIBOR (percent)		1.40%
Term Loans \| 2026 Term Loan \| Maximum \| LIBOR
Debt Instrument [Line Items]
Spread on LIBOR (percent)		2.50%
Term Loans \| 2026 Term Loan \| Maximum \| LIBOR \| External Credit Rating, Investment Grade
Debt Instrument [Line Items]
Spread on LIBOR (percent)		2.35%
Line of Credit \| Line of Credit
Debt Instrument [Line Items]
Credit facility	1,900,000,000
Revolving Credit Facility \| Revolving Credit Facility
Debt Instrument [Line Items]
Debt instrument extended term			1 year
Term of debt instrument			1 year
Revolving Credit Facility \| Revolving Credit Facility \| LIBOR
Debt Instrument [Line Items]
Spread on LIBOR (percent)			1.60%
Revolving Credit Facility \| Line of Credit
Debt Instrument [Line Items]
Covenant - minimum pool properties \| realEstateProperty		15	15
Covenant -maximum pool value contributed by single pool property of tenant (percent)		15.00%	15.00%
Covenant - maximum aggregate pool value to be contributed by pool properties subject to ground leases (percent)		15.00%	15.00%
Covenant - maximum aggregate pool value to be contributed by pool properties under development (percent)		20.00%	20.00%
Covenant - minimum aggregate leasing percentage (percent)		90.00%	90.00%
Covenant - maximum unsecured leverage ratio (percent)		60.00%	60.00%
Covenant - minimum unsecured interest coverage ratio		2.00	2.00
Revolving Credit Facility \| Unsecured Debt
Debt Instrument [Line Items]
Credit facility	$ 750,000,000
Debt instrument extended term	1 year
Additional increase in limit	$ 600,000,000
Remaining borrowing capacity		$ 376,500,000	$ 376,500,000
Revolving Credit Facility \| Unsecured Debt \| Minimum \| LIBOR
Debt Instrument [Line Items]
Spread on LIBOR (percent)		1.30%
Revolving Credit Facility \| Unsecured Debt \| Minimum \| LIBOR \| External Credit Rating, Investment Grade
Debt Instrument [Line Items]
Spread on LIBOR (percent)		0.825%
Revolving Credit Facility \| Unsecured Debt \| Maximum \| LIBOR
Debt Instrument [Line Items]
Spread on LIBOR (percent)		2.20%
Revolving Credit Facility \| Unsecured Debt \| Maximum \| LIBOR \| External Credit Rating, Investment Grade
Debt Instrument [Line Items]
Spread on LIBOR (percent)		1.55%