Debt Instrument, Amortization Period

No definition available.

Name:	gbdc_DebtInstrumentAmortizationPeriod
Namespace Prefix:	gbdc_
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

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Namespace Prefix:	gbdc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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

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Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
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- 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.

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- Definition

The fee, expressed as a percentage of the line of credit facility, for the line of credit facility regardless of whether the facility has been used.

+ References

No definition available.

+ Details

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Balance Type:	na
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- 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.

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Balance Type:	credit
Period Type:	instant

- Definition

Amount, after deduction of unamortized premium (discount) and debt issuance cost, of long-term debt. Excludes lease obligation.

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Namespace Prefix:	us-gaap_
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- Details

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Namespace Prefix:
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Balance Type:
Period Type:

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Balance Type:
Period Type:

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Period Type:

- Details

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Period Type:

- Details

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Period Type:

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Borrowings - JPM Credit Facility (Narrative) (Details) - USD ($)	Jun. 30, 2025	Apr. 04, 2025	Apr. 03, 2025	Feb. 11, 2021	Sep. 30, 2025	Dec. 06, 2024	Dec. 05, 2024	Sep. 30, 2024
Debt Instrument [Line Items]
Long-term debt					$ 4,900,773,000			$ 4,599,430,000
JPM Credit Facility \| Revolving Credit Facility \| Line of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity					1,997,500,000	$ 1,997,500,000	$ 1,822,500,000
Line of credit facility, commitment fee percentage		0.325%	0.375%	0.325%
Line of credit facility, maximum borrowing capacity		$ 3,000,000
Debt instrument, amortization period				1 year
Long-term debt					1,098,437,000			956,590,000
JPM Credit Facility \| Revolving Credit Facility \| Line of Credit \| Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate				0.10%
JPM Credit Facility \| Revolving Credit Facility \| Line of Credit \| SONIA
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate				0.0326%
JPM Credit Facility \| Revolving Credit Facility \| Line of Credit \| Canadian Overnight Repo Rate Average
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate				0.02955%
JPM Credit Facility \| Revolving Credit Facility \| Line of Credit \| Bank Bill Swap Rate
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate				0.20%
JPM Credit Facility \| Letter of Credit \| Line of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity				$ 23,750,000
Long-term debt					$ 0			$ 0
JPM Credit Facility - Term Benchmark Loan \| Revolving Credit Facility \| Line of Credit \| Base Rate
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	1.525%			1.65%
JPM Credit Facility - Term Benchmark Loan \| Revolving Credit Facility \| Line of Credit \| Base Rate \| Minimum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		1.525%		1.525%
JPM Credit Facility - Term Benchmark Loan \| Revolving Credit Facility \| Line of Credit \| Base Rate \| Maximum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		1.775%		1.775%
JPM Credit Facility- ABR Loan \| Revolving Credit Facility \| Line of Credit \| Base Rate
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	0.525%			0.65%
JPM Credit Facility- ABR Loan \| Revolving Credit Facility \| Line of Credit \| Base Rate \| Minimum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		0.525%		0.525%
JPM Credit Facility- ABR Loan \| Revolving Credit Facility \| Line of Credit \| Base Rate \| Maximum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		0.775%		0.775%