Gladstone Investment Corporation Quarterly report [Sections 13 or 15(d)]

SUBSEQUENT EVENTS - Narrative (Details) - USD ($) $ in Thousands		1 Months Ended	3 Months Ended		6 Months Ended
	Nov. 09, 2026	Oct. 31, 2026	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Subsequent Event [Line Items]
Success fee income			$ 0	$ 0	$ 5,591	$ 754
Purchase of investments					$ 157,093	$ 133,878
Subsequent Event
Subsequent Event [Line Items]
Success fee income		$ 500
Subsequent Event \| Specialized Fabrication Equipment Group LLC
Subsequent Event [Line Items]
Repayments of debt		54,600
Subsequent Event \| Smart Chemical Solutions, LLC \| Secured First Lien Debt
Subsequent Event [Line Items]
Purchase of investments		$ 3,700
Subsequent Event \| McFarlane Toys, Inc.
Subsequent Event [Line Items]
Purchase of investments	$ 56,100
Subsequent Event \| McFarlane Toys, Inc. \| Secured First Lien Debt
Subsequent Event [Line Items]
Purchase of investments	41,800
Subsequent Event \| McFarlane Toys, Inc. \| Preferred Equity
Subsequent Event [Line Items]
Purchase of investments	$ 14,300