Quarterly report [Sections 13 or 15(d)]

INVESTMENTS - Schedule of Fair Value Measurement Inputs and Valuation Techniques (Details)

v3.26.1
INVESTMENTS - Schedule of Fair Value Measurement Inputs and Valuation Techniques (Details)
$ in Thousands
Jun. 30, 2026
USD ($)
Sep. 30, 2025
USD ($)
investment
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments owned, fair value $ 953,326 $ 859,124 [1]
Investments at fair value 953,326 859,124
Gladstone Alternative    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value 4,900 5,100
Leeds Acquisition Holdings L L C    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value   36,000
Secured first lien debt    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value 665,834 622,371
Secured second lien debt    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value 185,844 150,542
Unsecured debt    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value 356 333
Significant Unobservable Inputs (Level 3)    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments owned, fair value 948,381 854,013
Investments at fair value 948,381 854,013
Significant Unobservable Inputs (Level 3) | Secured first lien debt    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value 665,834 622,371
Significant Unobservable Inputs (Level 3) | Secured second lien debt    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value 185,844 150,542
Significant Unobservable Inputs (Level 3) | Unsecured debt    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments at fair value 356 333
Significant Unobservable Inputs (Level 3) | Measurement Input, Expected Payoff Amount | Preferred and common equity / equivalents    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments owned, fair value   $ 2,200
Investment owned, number of investments | investment   1
Significant Unobservable Inputs (Level 3) | Yield Analysis | Secured first lien debt    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments owned, fair value 581,235 $ 548,670
Significant Unobservable Inputs (Level 3) | Yield Analysis | Secured second lien debt    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments owned, fair value $ 168,398 $ 141,750
Significant Unobservable Inputs (Level 3) | Yield Analysis | Discount Rate | Secured first lien debt | Minimum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investment owned, measurement input 0.095 0.094
Significant Unobservable Inputs (Level 3) | Yield Analysis | Discount Rate | Secured first lien debt | Maximum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investment owned, measurement input 0.200 0.198
Significant Unobservable Inputs (Level 3) | Yield Analysis | Discount Rate | Secured first lien debt | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investment owned, measurement input 0.113 0.117
Significant Unobservable Inputs (Level 3) | Yield Analysis | Discount Rate | Secured second lien debt | Minimum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investment owned, measurement input 0.106 0.114
Significant Unobservable Inputs (Level 3) | Yield Analysis | Discount Rate | Secured second lien debt | Maximum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investment owned, measurement input 0.138 0.152
Significant Unobservable Inputs (Level 3) | Yield Analysis | Discount Rate | Secured second lien debt | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investment owned, measurement input 0.120 0.131
Significant Unobservable Inputs (Level 3) | TEV | Secured first lien debt    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments owned, fair value $ 84,599 $ 73,701
Significant Unobservable Inputs (Level 3) | TEV | Secured second lien debt    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments owned, fair value 17,446 8,792
Significant Unobservable Inputs (Level 3) | TEV | Unsecured debt    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments owned, fair value 356 333
Significant Unobservable Inputs (Level 3) | TEV | Preferred and common equity / equivalents    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments owned, fair value $ 96,347 $ 80,767
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Secured first lien debt | Minimum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investment owned, measurement input 3.8 3.5
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Secured first lien debt | Maximum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investment owned, measurement input 8 8
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Secured first lien debt | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investment owned, measurement input 6.4 6
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Secured second lien debt | Minimum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investment owned, measurement input 5.2 5.4
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Secured second lien debt | Maximum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investment owned, measurement input 5.8 5.4
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Secured second lien debt | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investment owned, measurement input 5.5 5.4
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Unsecured debt | Minimum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investment owned, measurement input 8 8
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Unsecured debt | Maximum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investment owned, measurement input 8 8
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Unsecured debt | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investment owned, measurement input 8 8
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Preferred and common equity / equivalents | Minimum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investment owned, measurement input 3.8 3.5
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Preferred and common equity / equivalents | Maximum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investment owned, measurement input 15.1 14.6
Significant Unobservable Inputs (Level 3) | TEV | EBITDA multiple | Preferred and common equity / equivalents | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investment owned, measurement input 8.2 6.8
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Secured first lien debt | Minimum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments owned, fair value $ 367 $ 538
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Secured first lien debt | Maximum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments owned, fair value 4,459 4,655
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Secured first lien debt | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments owned, fair value 3,928 3,540
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Secured second lien debt | Minimum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments owned, fair value 2,116 2,345
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Secured second lien debt | Maximum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments owned, fair value 12,252 2,345
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Secured second lien debt | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments owned, fair value 7,926 2,345
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Unsecured debt | Minimum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments owned, fair value 3,868 3,804
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Unsecured debt | Maximum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments owned, fair value 3,868 3,804
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Unsecured debt | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments owned, fair value 3,868 3,804
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Preferred and common equity / equivalents | Minimum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments owned, fair value 367 538
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Preferred and common equity / equivalents | Maximum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments owned, fair value 151,055 142,549
Significant Unobservable Inputs (Level 3) | TEV | EBITDA | Preferred and common equity / equivalents | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments owned, fair value $ 27,433 $ 10,222
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Secured first lien debt | Minimum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investment owned, measurement input 0.6 0.6
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Secured first lien debt | Maximum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investment owned, measurement input 0.8 0.8
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Secured first lien debt | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investment owned, measurement input 0.7 0.7
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Unsecured debt | Minimum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investment owned, measurement input 0.9 0.9
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Unsecured debt | Maximum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investment owned, measurement input 0.9 0.9
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Unsecured debt | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investment owned, measurement input 0.9 0.9
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Preferred and common equity / equivalents | Minimum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investment owned, measurement input 0.6 0.6
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Preferred and common equity / equivalents | Maximum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investment owned, measurement input 0.9 0.9
Significant Unobservable Inputs (Level 3) | TEV | Revenue multiple | Preferred and common equity / equivalents | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investment owned, measurement input 0.6 0.7
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Secured first lien debt | Minimum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments owned, fair value $ 5,267 $ 10,844
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Secured first lien debt | Maximum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments owned, fair value 20,670 21,649
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Secured first lien debt | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments owned, fair value 12,472 15,090
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Unsecured debt | Minimum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments owned, fair value 4,264 4,846
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Unsecured debt | Maximum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments owned, fair value 4,264 4,846
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Unsecured debt | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments owned, fair value 4,264 4,846
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Preferred and common equity / equivalents | Minimum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments owned, fair value 4,264 4,846
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Preferred and common equity / equivalents | Maximum    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments owned, fair value 20,670 21,649
Significant Unobservable Inputs (Level 3) | TEV | Revenue | Preferred and common equity / equivalents | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Investments owned, fair value $ 5,960 $ 12,394
[1] Cumulative gross unrealized depreciation for federal income tax purposes is $59.7 million; cumulative gross unrealized appreciation for federal income tax purposes is $34.1 million. Cumulative net unrealized depreciation is $25.5 million, based on a tax cost of $884.7 million.