v3.26.3
Fair Value and Maturity of Debt Outstanding - Schedule of Carrying Values and Estimated Fair Values of Debt Instruments (Details) - USD ($)
$ in Thousands
Sep. 30, 2026
Jun. 30, 2026
Oct. 30, 2025
Sep. 30, 2021
May 27, 2021
Debt Instrument [Line Items]          
Principal Outstanding $ 1,537,158 $ 1,878,595      
Net Carrying Value 1,521,525 1,860,649      
Fair Value $ 1,463,676 1,816,957      
3.364% 2026 Notes          
Debt Instrument [Line Items]          
Stated rate 3.364%        
3.437% 2028 Notes          
Debt Instrument [Line Items]          
Stated rate 3.437%        
5.50% 2030 Notes          
Debt Instrument [Line Items]          
Stated rate 5.50%   5.50%    
Public Notes          
Debt Instrument [Line Items]          
Unamortized Discount & Debt Issuance Costs $ 8,668 10,547      
Prospect Capital InterNotes®          
Debt Instrument [Line Items]          
Unamortized Discount & Debt Issuance Costs 6,965 7,399      
Credit Facility | Revolving Credit Facility          
Debt Instrument [Line Items]          
Principal Outstanding 287,152 562,328      
Unamortized Discount & Debt Issuance Costs 12,940 14,128      
Net Carrying Value 287,152 562,328      
Fair Value $ 287,152 $ 562,328      
Effective Interest Rate 2.05% 2.05%      
Unsecured Debt | 3.364% 2026 Notes          
Debt Instrument [Line Items]          
Stated rate 3.364% 3.364%     3.364%
Principal Outstanding $ 263,220 $ 264,486      
Unamortized Discount & Debt Issuance Costs 164 435      
Net Carrying Value 263,056 264,051      
Fair Value $ 261,704 $ 261,169      
Effective Interest Rate 3.86% 3.89%      
Unsecured Debt | 3.437% 2028 Notes          
Debt Instrument [Line Items]          
Stated rate 3.437% 3.437%   3.437%  
Principal Outstanding $ 220,803 $ 254,749      
Unamortized Discount & Debt Issuance Costs 2,103 2,743      
Net Carrying Value 218,700 252,006      
Fair Value $ 205,367 $ 239,729      
Effective Interest Rate 3.92% 3.97%      
Unsecured Debt | 5.50% 2030 Notes          
Debt Instrument [Line Items]          
Stated rate 5.50% 5.50% 5.50%    
Principal Outstanding $ 167,755 $ 182,153      
Unamortized Discount & Debt Issuance Costs 6,401 7,369      
Net Carrying Value 161,354 174,784      
Fair Value $ 157,019 $ 165,650      
Effective Interest Rate 6.76% 6.76%      
Unsecured Debt | Public Notes          
Debt Instrument [Line Items]          
Principal Outstanding $ 651,778 $ 701,388      
Net Carrying Value 643,110 690,841      
Fair Value 624,090 666,548      
Unsecured Debt | Prospect Capital InterNotes®          
Debt Instrument [Line Items]          
Principal Outstanding 598,228 614,879      
Unamortized Discount & Debt Issuance Costs 6,965 7,399      
Net Carrying Value 591,263 607,480      
Fair Value $ 552,434 $ 588,081      
Effective Interest Rate 6.14% 6.04%