Quarterly report [Sections 13 or 15(d)]

INVESTMENTS - Schedule of Investments by Security Type at Fair Value and Investments Valued Using Level 3 Inputs (Details)

v3.26.3
INVESTMENTS - Schedule of Investments by Security Type at Fair Value and Investments Valued Using Level 3 Inputs (Details) - USD ($)
$ in Thousands
Sep. 30, 2026
Mar. 31, 2026
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value $ 1,430,865 $ 1,309,248
Cash equivalents 25 25
Total investments and cash equivalents, fair value 1,430,890 1,309,273
Fair Value    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 1,425,975 1,304,243
Quoted Prices in Active Markets for Identical Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 0 0
Cash equivalents 25 25
Total investments and cash equivalents, fair value 25 25
Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 0 0
Cash equivalents 0 0
Total investments and cash equivalents, fair value 0 0
Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 1,425,975 1,304,243
Cash equivalents 0 0
Total investments and cash equivalents, fair value 1,425,975 1,304,243
Significant Unobservable Inputs (Level 3) | Non-Control/Non-Affiliate Investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 1,024,751 983,959
Significant Unobservable Inputs (Level 3) | Affiliate Investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 342,508 319,671
Significant Unobservable Inputs (Level 3) | Control Investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 58,716 613
Fair Value Measured at Net Asset Value Per Share    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 4,890 5,005
Secured first lien debt    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 654,079 570,602
Secured first lien debt | Quoted Prices in Active Markets for Identical Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 0 0
Secured first lien debt | Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 0 0
Secured first lien debt | Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 654,079 570,602
Secured first lien debt | Significant Unobservable Inputs (Level 3) | Non-Control/Non-Affiliate Investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 422,961 374,285
Secured first lien debt | Significant Unobservable Inputs (Level 3) | Affiliate Investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 187,762 195,704
Secured first lien debt | Significant Unobservable Inputs (Level 3) | Control Investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 43,356 613
Secured second lien debt    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 107,333 99,197
Secured second lien debt | Quoted Prices in Active Markets for Identical Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 0 0
Secured second lien debt | Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 0 0
Secured second lien debt | Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 107,333 99,197
Secured second lien debt | Significant Unobservable Inputs (Level 3) | Non-Control/Non-Affiliate Investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 104,933 97,397
Secured second lien debt | Significant Unobservable Inputs (Level 3) | Affiliate Investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 2,400 1,800
Secured second lien debt | Significant Unobservable Inputs (Level 3) | Control Investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 0 0
Preferred equity    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 484,076 426,949
Preferred equity | Quoted Prices in Active Markets for Identical Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 0 0
Preferred equity | Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 0 0
Preferred equity | Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 484,076 426,949
Preferred equity | Significant Unobservable Inputs (Level 3) | Non-Control/Non-Affiliate Investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 316,370 304,782
Preferred equity | Significant Unobservable Inputs (Level 3) | Affiliate Investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 152,346 122,167
Preferred equity | Significant Unobservable Inputs (Level 3) | Control Investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 15,360 0
Common equity/equivalents    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 180,487 207,495
Common equity/equivalents | Gladstone Alternative    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 4,900 5,000
Common equity/equivalents | Quoted Prices in Active Markets for Identical Assets (Level 1)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 0 0
Common equity/equivalents | Significant Other Observable Inputs (Level 2)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 0 0
Common equity/equivalents | Significant Unobservable Inputs (Level 3)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 180,487 207,495
Common equity/equivalents | Significant Unobservable Inputs (Level 3) | Non-Control/Non-Affiliate Investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 180,487 207,495
Common equity/equivalents | Significant Unobservable Inputs (Level 3) | Affiliate Investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value 0 0
Common equity/equivalents | Significant Unobservable Inputs (Level 3) | Control Investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investments at fair value $ 0 $ 0