Incentive Fee Expense, Net Of Adjustments

No definition available.

Name:	gbdc_IncentiveFeeExpenseNetOfAdjustments
Namespace Prefix:	gbdc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Management And Service Fees, Cash Equivalents Maturity Period

+ References

No definition available.

+ Details

Name:	gbdc_ManagementAndServiceFeesCashEquivalentsMaturityPeriod
Namespace Prefix:	gbdc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Management And Service Fees, Number Of Components

+ References

No definition available.

+ Details

Name:	gbdc_ManagementAndServiceFeesNumberOfComponents
Namespace Prefix:	gbdc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of liabilities incurred and payable to vendors for goods and services received, and accrued liabilities classified as other.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-04(10)(d))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479170/946-210-S99-1

+ Details

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

- Definition

Amount of fee payable for management of operations based on performance under arrangement, including, but not limited to, investment.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 946
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-04(12)(a))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479170/946-210-S99-1

+ Details

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

- Definition

Amount of fee contractually waived by investment company.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480990/946-20-50-7

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07(2)(a))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479134/946-220-S99-1

+ Details

Name:	us-gaap_InvestmentCompanyContractualFeeWaived
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of carryover of excess expense potentially reimbursable to adviser but not recorded as liability by investment company.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480990/946-20-50-6

+ Details

Name:	us-gaap_InvestmentCompanyExcessExpenseReimbursable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22)(b))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(b))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

+ Details

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

- Definition

Base rate for management and service fees under arrangement to manage operations, including, but not limited to, investment.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07(2)(c))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479134/946-220-S99-1

+ Details

Name:	us-gaap_ManagementAndServiceFeesBaseRate
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Rate based on performance for management and service fees under arrangement to manage operations, including, but not limited to, investment.

+ References

+ Details

Name:	us-gaap_ManagementAndServiceFeesIncentiveRate
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of expense for investment management fee, including, but not limited to, expense in connection with research, selection, supervision, and custody of investment.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 946
-SubTopic 830
-Name Accounting Standards Codification
-Section 55
-Paragraph 10
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479168/946-830-55-10

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section 45
-Paragraph 3
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479105/946-220-45-3

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07(2)(a))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479134/946-220-S99-1

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-SubTopic 10
-Topic 850
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483326/850-10-50-1

+ Details

Name:	us-gaap_ManagementFeeExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 850
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483326/850-10-50-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 850
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483326/850-10-50-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 850
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483326/850-10-50-3

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480738/235-10-S50-1

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(k)(2))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480678/235-10-S99-1

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(k)(1))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480678/235-10-S99-1

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 13
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481990/310-10-45-13

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07(2)(c)(2)(i))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479134/946-220-S99-1

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07(2)(c)(2)(ii))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479134/946-220-S99-1

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07(2)(g)(3))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479134/946-220-S99-1

+ Details

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

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=us-gaap_RelatedPartyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	gbdc_InvestmentAdvisoryAgreementIncentiveFeeWaivedInExcessOfThresholdAxis=gbdc_InvestmentAdvisoryAgreementIncentiveFeeWaivedInExcessOfThresholdMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	gbdc_InvestmentAdvisoryAgreementIncentiveFeeWaivedInExcessOfThresholdAxis=gbdc_InvestmentAdvisoryAgreementIncentiveFeeCapWaivedInExcessOfThresholdMember
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_RelatedPartyTransactionAxis=gbdc_InvestmentAdvisoryAgreementIncentiveFeeCapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=gbdc_InvestmentAdvisoryAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=gbdc_InvestmentAdvisoryAgreementCumulativePreIncentiveFeeNetInvestmentIncomeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=gbdc_InvestmentAdvisoryAgreementIncomeIncentiveFeeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=gbdc_InvestmentManagementAgreementQuarterlyIncentiveRateCriteriaNotMetMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=gbdc_InvestmentManagementAgreementIncentiveRatePreIncentiveFeeNetInvestmentIncomeBelowCatchUpMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=gbdc_InvestmentManagementAgreementIncentiveRateQuarterlyCatchUpThresholdMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=gbdc_InvestmentAdvisoryAgreementCapitalGainIncentiveFeeBaseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=gbdc_AdministrationAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=gbdc_ExpensesPaidMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=gbdc_InvestmentManagementAgreementIncentiveRateQuarterlyHurdleRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Related Party Transactions - Narrative (Details)					3 Months Ended		12 Months Ended			15 Months Ended
Related Party Transactions - Narrative (Details)	Jan. 16, 2024	Jan. 15, 2024	Jul. 01, 2023 component	Jun. 30, 2023	Sep. 30, 2024 USD ($)	Jun. 30, 2024 USD ($)	Sep. 30, 2025 USD ($)	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Jun. 30, 2025	Jun. 03, 2024 USD ($)
Related Party Transaction [Line Items]
Base management fee							$ 87,891,000	$ 62,514,000	$ 70,802,000
Incentive fee							73,282,000	77,163,000	74,066,000
Incentive fee payable					$ 0		0	0
Accounts payable and other liabilities					10,299,000		9,759,000	$ 10,299,000
Related Party
Related Party Transaction [Line Items]
Management and service fees, incentive rate	15.00%							20.00%
Related Party \| Investment Advisory Agreement, Incentive Fee Threshold
Related Party Transaction [Line Items]
Management and service fees, incentive rate	15.00%
Related Party \| Investment Advisory Agreement, Incentive Fee Cap Threshold
Related Party Transaction [Line Items]
Management and service fees, incentive rate	15.00%
Related Party \| Revolving Credit Facility
Related Party Transaction [Line Items]
Line of credit facility, maximum borrowing capacity					200,000,000		300,000,000	$ 200,000,000			$ 100,000,000
Investment Advisory Agreement, Incentive Fee Cap \| Related Party
Related Party Transaction [Line Items]
Management and service fees, incentive rate	15.00%	20.00%
Investment Advisory Agreement \| Related Party
Related Party Transaction [Line Items]
Management and service fees, base rate			1.00%	1.375%
Management and service fees, number of fee components \| component			2
Management and service fees, cash equivalents maturity period	270 days
Investment company, excess expense reimbursable							8,347,000	12,959,000	8,158,000
Cumulative Pre-Incentive Fee Net Investment Income \| Related Party
Related Party Transaction [Line Items]
Management and service fees, incentive rate										15.00%
Incentive fee							0	0	0
Investment Advisory Agreement, Income Incentive Fee \| Related Party
Related Party Transaction [Line Items]
Base management fee waived					7,767,000	$ 7,767,000	$ 0	27,324,000	$ 0
Investment Advisory Agreement, Income Incentive Fee \| Related Party \| Investment Advisory Agreement, Incentive Fee Threshold
Related Party Transaction [Line Items]
Base management fee waived								$ 5,157,000
Investment Management Agreement - Quarterly Incentive Rate, Criteria Not Met \| Related Party
Related Party Transaction [Line Items]
Management and service fees, incentive rate								0.00%		0.00%
Investment Management Agreement - Incentive Rate, Pre-Incentive Fee Net Investment Income Below Catch-Up \| Related Party
Related Party Transaction [Line Items]
Management and service fees, incentive rate								100.00%		100.00%
Investment Management Agreement - Incentive Rate, Quarterly Catch-Up Threshold \| Related Party
Related Party Transaction [Line Items]
Management and service fees, incentive rate								2.50%
Capital Gain Incentive Fee Base \| Related Party
Related Party Transaction [Line Items]
Management and service fees, incentive rate							15.00%
Administration Agreement \| Related Party
Related Party Transaction [Line Items]
Accounts payable and other liabilities					2,840,000		$ 3,022,000	$ 2,840,000
Expenses Paid \| Related Party
Related Party Transaction [Line Items]
Accounts payable and other liabilities					$ 2,658,000		$ 2,527,000	$ 2,658,000
Investment Management Agreement - Incentive Rate, Quarterly Hurdle Rate \| Related Party
Related Party Transaction [Line Items]
Management and service fees, incentive rate							2.00%