Gladstone Capital Corporation Annual report [Section 13 and 15(d), not S-K Item 405]

SUBSEQUENT EVENTS - Schedule of Subsequent Events (Details) - $ / shares		12 Months Ended
	Oct. 14, 2025	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Subsequent Event [Line Items]
Preferred stock, dividends, declared, ordinary (in USD per share)		$ (0.04)	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0
Subsequent Event
Subsequent Event [Line Items]
Common stock, dividends, declared, ordinary (in USD per share)	$ 0.45
Preferred stock, dividends, declared, ordinary (in USD per share)	0.390624
O 2024 M1 Dividends \| Subsequent Event
Subsequent Event [Line Items]
Common stock, dividends, declared, ordinary (in USD per share)	0.15
Preferred stock, dividends, declared, ordinary (in USD per share)	0.130208
O 2024 M2 Dividends \| Subsequent Event
Subsequent Event [Line Items]
Common stock, dividends, declared, ordinary (in USD per share)	0.15
Preferred stock, dividends, declared, ordinary (in USD per share)	0.130208
O 2024 M3 Dividends \| Subsequent Event
Subsequent Event [Line Items]
Common stock, dividends, declared, ordinary (in USD per share)	0.15
Preferred stock, dividends, declared, ordinary (in USD per share)	$ 0.130208