Annual report pursuant to Section 13 and 15(d)

Related-Party Transactions - Summary of Management Fees, Incentive Fees and Associated Credits and Administration Fees (Detail)

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Related-Party Transactions - Summary of Management Fees, Incentive Fees and Associated Credits and Administration Fees (Detail) - USD ($)
 3 Months Ended 12 Months Ended
Dec. 31, 2023
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Administration fee        
Related Party Transaction [Line Items]        
Total fees to Gladstone Securities   $ 2,255,000 $ 2,005,000 $ 1,526,000
Adviser        
Related Party Transaction [Line Items]        
Total fees to Gladstone Securities   10,374,000 11,532,000 10,230,000
Adviser | Base management fee        
Related Party Transaction [Line Items]        
Total fees to Gladstone Securities   8,603,000 8,307,000 6,329,000
Adviser | Incentive fee        
Related Party Transaction [Line Items]        
Total fees to Gladstone Securities   1,771,000 3,225,000 3,901,000
Gladstone Securities | Financing Fees and Selling Commissions and Dealer Management Fees        
Related Party Transaction [Line Items]        
Total fees to Gladstone Securities   583,000 13,625,000 5,127,000
Gladstone Securities | Selling Commissions and Dealer Management Fees        
Related Party Transaction [Line Items]        
Total fees to Gladstone Securities $ 583,000 583,000 13,471,000 4,961,000
Gladstone Securities | Financing Fees        
Related Party Transaction [Line Items]        
Total fees to Gladstone Securities   $ 0 $ 154,000 $ 166,000