v3.26.1

SUBSEQUENT EVENTS - Narrative (Details) - USD ($) $ in Thousands	1 Months Ended	6 Months Ended
	Apr. 30, 2026	Mar. 31, 2026	Mar. 31, 2025	Sep. 30, 2025
Subsequent Event [Line Items]
Purchase of investments		$ 142,724	$ 197,211
Committed capital, unfunded		$ 72,364		$ 66,396
OneSource HoldCo, LLC \| Subsequent Event
Subsequent Event [Line Items]
Purchase of investments	$ 12,700
Financial support to investee contractually required	2,000
Financial support to investee funded	1,000
SWECO Worldwide, Inc \| Subsequent Event
Subsequent Event [Line Items]
Purchase of investments	32,500
Committed capital, unfunded	$ 6,000

Investment Company, Financial Support To Investee Funded

+ References

No definition available.

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- Definition

Amount of financial support committed by investment company to investee that is contractually required but not provided.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 16
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480990/946-20-50-16

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- Definition

Amount of financial support committed by investment company to investee that is contractually required.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 15
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480990/946-20-50-15

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 15
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480990/946-20-50-15

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- Definition

Amount of cash outflow for acquisition of investment, classified as operating activity. Includes, but is not limited to, funding, origination, drawdown, and participation in investment.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 946
-SubTopic 230
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477867/946-230-45-1

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- Definition

Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 830
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481674/830-30-50-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 855
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483399/855-10-50-2

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GLADSTONE CAPITAL CORP 10-Q