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

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

BORROWINGS - Notes Payable and Convertible Notes Narrative (Details)

BORROWINGS - Notes Payable and Convertible Notes Narrative (Details) $ / shares in Units, $ in Thousands				1 Months Ended					12 Months Ended
	Oct. 31, 2025 USD ($)	Oct. 15, 2025 USD ($)	Sep. 22, 2025 $ / shares	Sep. 30, 2025 USD ($) day $ / shares	Aug. 31, 2023 USD ($)	Nov. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2025 USD ($) $ / shares	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Apr. 30, 2022	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)
Debt Instrument [Line Items]
Aggregate amount				$ 406,500					$ 406,500	$ 257,000	$ 257,000	$ 200,000		$ 188,813	$ 96,313	$ 57,500	$ 0	$ 0	$ 0
Proceeds from issuance of notes payable									147,258	0	$ 57,000
Interest payable				3,141					3,141	$ 2,916
Notes Payable \| 7.75% Notes due 2028
Debt Instrument [Line Items]
Interest rate					7.75%
Aggregate amount					$ 57,000
Proceeds from issuance of notes payable					$ 55,100
Notes payable, fair value disclosure				57,300					57,300
Notes Payable \| 7.75% Notes due 2028 \| Subsequent Event
Debt Instrument [Line Items]
Aggregate amount		$ 57,000
Percentage of principal amount redeemed		100.00%
Notes Payable \| 3.75% Notes due 2027
Debt Instrument [Line Items]
Interest rate						3.75%							3.75%
Aggregate amount						$ 50,000
Proceeds from issuance of notes payable						48,500
Notes payable, fair value disclosure				48,000					48,000
Interest payable						$ 1,900
Notes Payable \| 5.125% Notes due 2026
Debt Instrument [Line Items]
Interest rate							5.125%	5.125%
Aggregate amount							$ 50,000	$ 100,000
Proceeds from issuance of notes payable							$ 50,600	$ 97,700
Notes payable, fair value disclosure				$ 149,400					$ 149,400
Notes Payable \| 5.125% Notes due 2026 \| Subsequent Event
Debt Instrument [Line Items]
Aggregate amount	$ 150,000
Percentage of principal amount redeemed	100.00%
Convertible Debt \| 2030 Convertible Notes
Debt Instrument [Line Items]
Interest rate				5.875%					5.875%
Aggregate principal amount				$ 149,500					$ 149,500
Proceeds from convertible debt				142,800
Interest payable				$ 8,800
Conversion rate			0.03861	0.0384394
Conversion price (in dollars per share) \| $ / shares			$ 25.90	$ 26.02					$ 26.02
Threshold percentage of stock price trigger				130.00%
Threshold trading days \| day				20
Threshold consecutive trading days \| day				30
Redemption price				100.00%
Convertible debt, fair value				$ 146,900					$ 146,900
Convertible Debt \| 2030 Convertible Notes \| Maximum
Debt Instrument [Line Items]
Conversion rate				0.0422834