Annual report pursuant to Section 13 and 15(d)


Cover - USD ($)
12 Months Ended
Sep. 30, 2023
Nov. 10, 2023
Mar. 31, 2023
Document Information [Line Items]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Sep. 30, 2023    
Current Fiscal Year End Date --09-30    
Document Transition Report false    
Securities Act File Number 814-00237    
Entity Registrant Name GLADSTONE CAPITAL CORP    
Entity Incorporation, State or Country Code MD    
Entity Tax Identification Number 54-2040781    
Entity Address, Address Line One 1521 Westbranch Drive    
Entity Address, Address Line Two Suite 100    
Entity Address, City or Town McLean    
Entity Address, State or Province VA    
Entity Address, Postal Zip Code 22102    
City Area Code 703    
Local Phone Number 287-5800    
Title of 12(g) Security 6.25% Series A Cumulative Redeemable Preferred Stock    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Non-accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag false    
Entity Shell Company false    
Entity Public Float     $ 332,891,314
Entity Common Stock, Shares Outstanding   43,508,897  
Documents Incorporated by Reference Documents Incorporated by Reference. Portions of the Registrant’s definitive proxy statement to be filed with the Securities and Exchange Commission pursuant to Regulation 14A in connection with the Registrant’s 2024 Annual Meeting of Stockholders, which will be filed subsequent to the date hereof, are incorporated by reference into Part III of this Form 10-K. Such proxy statement will be filed with the Securities and Exchange Commission not later than 120 days following the end of the Registrant’s fiscal year ended September 30, 2023.    
Document Fiscal Year Focus 2023    
Entity Central Index Key 0001143513    
Amendment Flag false    
Document Fiscal Period Focus FY    
Document Financial Statement Error Correction [Flag] false    
Common Stock      
Document Information [Line Items]      
Title of 12(b) Security Common Stock, $0.001 par value per share    
Trading Symbol GLAD    
Security Exchange Name NASDAQ    
7.75% Notes due 2028      
Document Information [Line Items]      
Title of 12(b) Security 7.75% notes due 2028    
Trading Symbol GLADZ    
Security Exchange Name NASDAQ