TENANT ORIGINATION AND ABSORPTION COSTS, ABOVE-MARKET LEASE ASSETS AND BELOW-MARKET LEASE LIABILITIES (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Tenant Origination and Absorption Costs, Above-Market Lease Assets and Below-Market Lease Liabilities [Abstract]
Tenant Origination And Absorption Costs, Cost	$ 11,875		$ 13,822
Tenant Origination and Absorption Costs, Accumulated Amortization	(5,696)		(6,818)
Tenant Origination and Absorption Costs, Net Amount	6,179		7,004
Tenant Origination and Absorption Costs, Amortization	(571)	$ (690)
Above-Market Lease Assets, Cost	213		213
Above-Market Lease Assets, Accumulated Amortization	(100)		(93)
Above-Market Lease Assets, Net Amount	113		120
Above-Market Lease Assets, Amortization	(7)	(7)
Below-Market Lease Liabilities, Cost	(5,930)		(6,213)
Below-Market Lease Liabilities, Accumulated Amortization	3,682		3,680
Below-Market Lease Liabilities, Net Amount	(2,248)		$ (2,533)
Below-Market Lease Liabilities, Amortization	$ 285	$ 350

Above-market leases, accumulated amortization.

No definition available.

Name:	kbsgi_AboveMarketLeasesAccumulatedAmortization
Namespace Prefix:	kbsgi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Above-market leases, amortization expense.

+ References

No definition available.

+ Details

Name:	kbsgi_AboveMarketLeasesAmortizationExpense
Namespace Prefix:	kbsgi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Above-market leases, gross.

+ References

No definition available.

+ Details

Name:	kbsgi_AboveMarketLeasesGross
Namespace Prefix:	kbsgi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This element represents an asset associated with the acquisition of an off-market lease when the terms of the lease are favorable to the market terms for the lease at the date of acquisition.

+ References

No definition available.

+ Details

Name:	kbsgi_AboveMarketLeasesNet
Namespace Prefix:	kbsgi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Off-market lease, unfavorable, amortization expense.

+ References

No definition available.

+ Details

Name:	kbsgi_OffMarketLeaseUnfavorableAmortizationExpense
Namespace Prefix:	kbsgi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Tenant Origination and Absorption Costs, Above-Market Lease Assets and Below-Market Lease Liabilities [Abstract]

+ References

No definition available.

+ Details

Name:	kbsgi_TenantOriginationAndAbsorptionCostsAboveMarketLeaseAssetsAndBelowMarketLeaseLiabilitiesAbstract
Namespace Prefix:	kbsgi_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Tenant origination and absorption costs, accumulated Amortization

+ References

No definition available.

+ Details

Name:	kbsgi_TenantOriginationAndAbsorptionCostsAccumulatedAmortization
Namespace Prefix:	kbsgi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Tenant origination and absorption costs, amortization expense.

+ References

No definition available.

+ Details

Name:	kbsgi_TenantOriginationAndAbsorptionCostsAmortizationExpense
Namespace Prefix:	kbsgi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the net amount of tenant origination and absorption costs. Origination costs include estimates of costs avoided associated with leasing the property, including tenant allowance and improvements and leasing commissions. Absorption costs relate to the value of opportunity costs calculated using the contractual amounts paid pursuant to in-place leases over market absorption period for a similar lease.

+ References

No definition available.

+ Details

Name:	kbsgi_TenantOriginationAndAbsorptionCostsGross
Namespace Prefix:	kbsgi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Tenant origination and absorption costs, net.

+ References

No definition available.

+ Details

Name:	kbsgi_TenantOriginationAndAbsorptionCostsNet
Namespace Prefix:	kbsgi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated amortization of leases acquired as part of a real property acquisition at below market lease rate with a finite life.

+ References

No definition available.

+ Details

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

- Definition

Amount before amortization of leases acquired as part of a real property acquisition at below market lease rate with a finite life.

+ References

No definition available.

+ Details

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

- Definition

Amount after amortization of leases acquired as part of a real property acquisition at below market lease rate with a finite life.

+ References

No definition available.

+ Details

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