N-2	9 Months Ended
N-2	Jun. 30, 2022
Cover [Abstract]
Entity Central Index Key	0001476765
Amendment Flag	false
Securities Act File Number	814-00794
Document Type	10-Q
Entity Registrant Name	Golub Capital BDC, Inc.
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
Entity Emerging Growth Company	false
General Description of Registrant [Abstract]
Investment Objectives and Practices [Text Block]	Our investment objective is to generate current income and capital appreciation by investing primarily in one stop (a loan that combines characteristics of traditional first lien senior secured loans and second lien or subordinated loans and that are often referred to by other middle-market lenders as unitranche loans) and other senior secured loans of U.S. middle-market companies. We also selectively invest in second lien and subordinated loans of, and warrants and minority equity securities in U.S. middle-market companies. We intend to achieve our investment objective by (1) accessing the established loan origination channels developed by Golub Capital, a leading lender to U.S. middle-market companies with over $50.0 billion in capital under management as of April 1, 2022, (2) selecting investments within our core middle-market company focus, (3) partnering with experienced private equity firms, or sponsors, in many cases with whom Golub Capital has invested alongside in the past, (4) implementing the disciplined underwriting standards of Golub Capital and (5) drawing upon the aggregate experience and resources of Golub Capital.
Risk Factors [Table Text Block]	Item 1A: Risk Factors. You should carefully consider the risk factors set forth below and those described in Part I, Item 1A. Risk Factors in our annual report on Form 10-K for the year ended September 30, 2021 and in our quarterly report on Form 10-Q for the quarter ended March 31, 2022, together with all of the other information included in this report. The risks set out below, in our annual report on Form 10-K for the fiscal year ended September 30, 2021 and in our quarterly report on Form 10-Q for the quarter ended March 31, 2022 are not the only risks we face. Additional risks and uncertainties not presently known to us or not presently deemed material by us could also impair our operations and performance. If any of these events occur, our business, financial condition, results of operations and cash flows could be materially and adversely affected. In such case, our net asset value and the trading price of our common stock could decline, and you could lose all or part of your investment. The risk factors described below and in our annual report on Form 10-K for the fiscal year ended September 30, 2021 and in our quarterly report on Form 10-Q for the quarter ended March 31, 2022 are the principal risk factors associated with an investment in us as well as those factors generally associated with an investment company with investment objectives, investment policies, capital structure or trading markets similar to ours. Inflation may adversely affect the business, results of operations and financial condition of our portfolio companies. Certain of our portfolio companies are in industries that may be impacted by inflation. If such portfolio companies are unable to pass any increases in their costs of operations along to their customers, it could adversely affect their operating results and impact their ability to pay interest and principal on our loans, particularly if interest rates rise in response to inflation. In addition, any projected future decreases in our portfolio companies’ operating results due to inflation could adversely impact the fair value of those investments. Any decreases in the fair value of our investments could result in future realized or unrealized losses and therefore reduce our net assets resulting from operations.
Inflation Risk [Member]
General Description of Registrant [Abstract]
Risk [Text Block]	Inflation may adversely affect the business, results of operations and financial condition of our portfolio companies. Certain of our portfolio companies are in industries that may be impacted by inflation. If such portfolio companies are unable to pass any increases in their costs of operations along to their customers, it could adversely affect their operating results and impact their ability to pay interest and principal on our loans, particularly if interest rates rise in response to inflation. In addition, any projected future decreases in our portfolio companies’ operating results due to inflation could adversely impact the fair value of those investments. Any decreases in the fair value of our investments could result in future realized or unrealized losses and therefore reduce our net assets resulting from operations.