Annual report pursuant to Section 13 and 15(d)

RELATED-PARTY TRANSACTIONS - Schedule of Management Fees, Incentive Fees and Associated Credits and Administration Fees (Details)

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RELATED-PARTY TRANSACTIONS - Schedule of Management Fees, Incentive Fees and Associated Credits and Administration Fees (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Administration fee      
Related Party Transaction [Line Items]      
Total fees to Gladstone Securities $ 2,452 $ 2,255 $ 2,005
Financing Fees      
Related Party Transaction [Line Items]      
Total fees to Gladstone Securities 0 0 154
Adviser      
Related Party Transaction [Line Items]      
Total fees to Gladstone Securities 8,370 10,374 11,532
Adviser | Base management fee      
Related Party Transaction [Line Items]      
Total fees to Gladstone Securities 8,370 8,603 8,307
Adviser | Incentive fee      
Related Party Transaction [Line Items]      
Total fees to Gladstone Securities 109 1,771 3,225
Adviser | Incentive fee waiver      
Related Party Transaction [Line Items]      
Total fees to Gladstone Securities (109) 0 0
Gladstone Securities | Financing Fees and Selling Commissions and Dealer Management Fees      
Related Party Transaction [Line Items]      
Total fees to Gladstone Securities 41 583 13,625
Gladstone Securities | Selling Commissions and Dealer Management Fees      
Related Party Transaction [Line Items]      
Total fees to Gladstone Securities 41 583 13,471
Gladstone Securities | Financing Fees      
Related Party Transaction [Line Items]      
Total fees to Gladstone Securities $ 0 $ 0 $ 154