Acquisition of Golub Capital BDC 3, Inc. - Narrative (Details) - $ / shares	Jun. 03, 2024	Sep. 30, 2025	Feb. 21, 2025	Sep. 30, 2024	Oct. 06, 2023
Business Combination, Separately Recognized Transaction [Line Items]
Common stock, par value (in dollars per share)		$ 0.001	$ 0.001	$ 0.001	$ 0.001
GBDC 3
Business Combination, Separately Recognized Transaction [Line Items]
Cash per share for GBDC 3 common stock converted (in dollars per share)	$ 0.9138
Common stock, par value (in dollars per share)	$ 0.001
Number of shares issued (in shares)	92,115,308