Cover - shares	6 Months Ended
	Mar. 31, 2023	May 08, 2023
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2023
Document Transition Report	false
Securities Act File Number	814-00794
Entity Registrant Name	Golub Capital BDC, Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	27-2326940
Entity Address, Address Line One	200 Park Avenue
Entity Address, Address Line Two	25th Floor
Entity Address, City or Town	New York
Entity Address, State or Province	NY
Entity Address, Postal Zip Code	10166
City Area Code	212
Local Phone Number	750-6060
Title of 12(b) Security	Common Stock, par value $0.001 per share
Trading Symbol	GBDC
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		169,963,496
Amendment Flag	false
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	Q2
Entity Central Index Key	0001476765
Current Fiscal Year End Date	--09-30