Common share, distribution rate per share per day, declared.

No definition available.

Name:	kbsgi_CommonShareDistributionRatePerSharePerDayDeclared
Namespace Prefix:	kbsgi_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

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

- Definition

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

+ Details

Name:	us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Aggregate dividends declared during the period for each share of common stock outstanding.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=120397183&loc=d3e187085-122770

+ Details

Name:	us-gaap_CommonStockDividendsPerShareDeclared
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
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_EarningsPerShareBasicLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
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=kbsgi_CommonClassTMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Per Share Data (Details) - $ / shares	3 Months Ended	4 Months Ended	5 Months Ended	6 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	May 31, 2019	Jun. 30, 2020	Sep. 30, 2020	Sep. 30, 2019
Accounting Policies [Abstract]
Potentially dilutive securities					0	0
Class A Common Stock
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Distributions declared per share (in dollars per share)	$ 0.13800000				$ 0.12645000	$ 0.41236183
Distribution rate per share per day, declared (in dollars per share)		$ 0.046	$ 0.00151233	$ 0.02107500
Class T Common Stock
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Distributions declared per share (in dollars per share)	$ 0.13800000				$ 0.12645000	$ 0.41236183
Distribution rate per share per day, declared (in dollars per share)		$ 0.046	$ 0.00151233	$ 0.02107500