Acquisition of Golub Capital BDC 3, Inc. - Schedule of Recognized Identified Assets Acquired and Liabilities Assumed (Details) - USD ($)	Jun. 03, 2024	Sep. 30, 2025	Sep. 30, 2024
Liabilities assumed:
Investments, amortization cost	$ 2,675,051,000
Incentive fee payable		$ 0	$ 0
GBDC 3
Business Combination, Separately Recognized Transaction [Line Items]
Common stock issued by the Company	1,526,351,000
Transaction costs	3,867,000
Total purchase price	1,530,218,000
Assets acquired:
Investments, at fair value (amortized cost of $2,675,051)	2,623,381,000
Cash and cash equivalents	53,885,000
Interest receivable	32,204,000
Other assets	15,410,000
Total assets acquired	2,724,880,000
Liabilities assumed:
Debt	1,211,174,000
Other liabilities	35,156,000
Total liabilities assumed	1,246,330,000
Net assets acquired	1,478,550,000
Total purchase premium	$ 51,668,000
Share price (in dollars per share)	$ 16.57
Number of shares issued (in shares)	92,115,308
Management fees payable	$ 4,651,000
Incentive fee payable	4,271,000
GBDC 3 | Restricted Cash and Cash Equivalents
Assets acquired:
Cash and cash equivalents	46,067,000
GBDC 3 | Foreign Restricted Cash and Cash Equivalents
Assets acquired:
Cash and cash equivalents	373,000
GBDC 3 | Foreign Cash and Cash Equivalents
Assets acquired:
Cash and cash equivalents	$ 1,064,000