v3.26.1
Financial Highlights (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 9 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Mar. 31, 2026
Sep. 30, 2025
Mar. 31, 2025
Sep. 30, 2024
Investment Company, Financial Highlights [Roll Forward]                
Number of common shares outstanding (in shares) 185,108,156   185,108,156     161,326,430    
Net assets at end of period $ 4,474,916 $ 3,039,334 $ 4,474,916 $ 3,039,334 $ 4,549,233 $ 4,059,469 $ 2,452,563 $ 1,732,342
Asset coverage ratio 183.60%   183.60%          
2025 Debt Securitization                
Investment Company, Financial Highlights [Roll Forward]                
Average debt outstanding $ 599,090 0 $ 599,090 0        
2026-B Debt Securitization                
Investment Company, Financial Highlights [Roll Forward]                
Average debt outstanding 335,000 0 206,154 0        
BANA Credit Facility                
Investment Company, Financial Highlights [Roll Forward]                
Average debt outstanding 386,818 97,626 446,366 32,542        
2028 Notes                
Investment Company, Financial Highlights [Roll Forward]                
Average debt outstanding 500,000 0 500,000 0        
2029 Notes                
Investment Company, Financial Highlights [Roll Forward]                
Average debt outstanding 500,000 500,000 500,000 500,000        
2030 Notes                
Investment Company, Financial Highlights [Roll Forward]                
Average debt outstanding 500,000 500,000 500,000 232,601        
2031 Notes                
Investment Company, Financial Highlights [Roll Forward]                
Average debt outstanding 500,000 0 280,220 0        
Other Short-Term Borrowings                
Investment Company, Financial Highlights [Roll Forward]                
Average debt outstanding $ 141,699 $ 4,263 $ 136,995 $ 1,421        
Common Class I                
Investment Company, Financial Highlights [Roll Forward]                
Net asset value at beginning of period (in dollars per share)     $ 25.16 $ 25.10        
Distributions declared (in dollars per share)     (1.69) (1.73)        
Net investment income - after tax (in dollars per share)     1.65 1.64        
Net realized gain (loss) on investment transactions (in dollars per share)     (0.11) (0.02)        
Net change in unrealized appreciation (depreciation) on investment transactions (in dollars per share)     (0.84) 0.19        
Distribution and shareholder servicing fees (in dollars per share)     0 0        
Net asset value at end of period (in dollars per share) $ 24.17 $ 25.18 $ 24.17 $ 25.18        
Total return based on net asset value per share     2.89% 7.64%        
Number of common shares outstanding (in shares) 176,261,880 114,728,035 176,261,880 114,728,035   153,882,766    
Ratio of net investment income - after tax to average net assets     8.90% 8.82%        
Ratio of total expenses to average net assets     10.14% 10.23%        
Ratio of incentive fees to average net assets     0.86% 1.04%        
Ratio of net expenses to average net assets     10.14% 10.23%        
Ratio of total expenses (without incentive fees) to average net assets     9.28% 9.19%        
Total return based on average net asset value     2.71% 7.27%        
Total return based on average net asset value - annualized     3.62% 9.72%        
Net assets at end of period $ 4,261,108 $ 2,888,342 $ 4,261,108 $ 2,888,342   $ 3,872,168    
Average debt outstanding     $ 5,506,949 $ 2,321,894        
Average debt outstanding per share (in dollars per share)     $ 30.41 $ 24.70        
Portfolio Turnover     20.00% 10.40%        
Asset coverage ratio 183.57% 182.47% 183.57% 182.47%        
Asset coverage ratio per unit (in dollars per share) $ 1,836 $ 1,825 $ 1,836 $ 1,825        
Common Class I | 2025 Debt Securitization                
Investment Company, Financial Highlights [Roll Forward]                
Average market value per unit (in dollars per share)   0   0        
Common Class I | 2026-B Debt Securitization                
Investment Company, Financial Highlights [Roll Forward]                
Average market value per unit (in dollars per share)   0   0        
Common Class I | BANA Credit Facility                
Investment Company, Financial Highlights [Roll Forward]                
Average market value per unit (in dollars per share)   0   0        
Common Class I | 2028 Notes                
Investment Company, Financial Highlights [Roll Forward]                
Average market value per unit (in dollars per share) 999 0 999 0        
Common Class I | 2029 Notes                
Investment Company, Financial Highlights [Roll Forward]                
Average market value per unit (in dollars per share) 993 987 993 987        
Common Class I | 2030 Notes                
Investment Company, Financial Highlights [Roll Forward]                
Average market value per unit (in dollars per share) 1,001 996 1,001 996        
Common Class I | 2031 Notes                
Investment Company, Financial Highlights [Roll Forward]                
Average market value per unit (in dollars per share) 967 0 967 0        
Common Class I | Other Short-Term Borrowings                
Investment Company, Financial Highlights [Roll Forward]                
Average market value per unit (in dollars per share)   0   0        
Common Class S                
Investment Company, Financial Highlights [Roll Forward]                
Net asset value at beginning of period (in dollars per share)     25.16 25.10        
Distributions declared (in dollars per share)     (1.53) (1.57)        
Net investment income - after tax (in dollars per share)     1.65 1.64        
Net realized gain (loss) on investment transactions (in dollars per share)     (0.11) (0.02)        
Net change in unrealized appreciation (depreciation) on investment transactions (in dollars per share)     (0.84) 0.19        
Distribution and shareholder servicing fees (in dollars per share)     (0.16) (0.16)        
Net asset value at end of period (in dollars per share) $ 24.17 $ 25.18 $ 24.17 $ 25.18        
Total return based on net asset value per share     2.12% 6.74%        
Number of common shares outstanding (in shares) 8,846,276 5,997,598 8,846,276 5,997,598   7,443,664    
Ratio of net investment income - after tax to average net assets     8.06% 8.00%        
Ratio of total expenses to average net assets     11.00% 11.07%        
Ratio of incentive fees to average net assets     0.86% 1.04%        
Ratio of net expenses to average net assets     11.00% 11.07%        
Ratio of total expenses (without incentive fees) to average net assets     10.14% 10.03%        
Total return based on average net asset value     2.13% 6.64%        
Total return based on average net asset value - annualized     2.85% 8.88%        
Net assets at end of period $ 213,808 $ 150,992 $ 213,808 $ 150,992   $ 187,302    
Average debt outstanding     $ 5,506,949 $ 2,321,894        
Average debt outstanding per share (in dollars per share)     $ 30.41 $ 24.70        
Portfolio Turnover     20.00% 10.40%        
Asset coverage ratio 183.57% 182.47% 183.57% 182.47%        
Asset coverage ratio per unit (in dollars per share) $ 1,836 $ 1,825 $ 1,836 $ 1,825        
Common Class S | 2025 Debt Securitization                
Investment Company, Financial Highlights [Roll Forward]                
Average market value per unit (in dollars per share)   0   0        
Common Class S | 2026-B Debt Securitization                
Investment Company, Financial Highlights [Roll Forward]                
Average market value per unit (in dollars per share)   0   0        
Common Class S | BANA Credit Facility                
Investment Company, Financial Highlights [Roll Forward]                
Average market value per unit (in dollars per share)   0   0        
Common Class S | 2028 Notes                
Investment Company, Financial Highlights [Roll Forward]                
Average market value per unit (in dollars per share) 999 0 999 0        
Common Class S | 2029 Notes                
Investment Company, Financial Highlights [Roll Forward]                
Average market value per unit (in dollars per share) 993 987 993 987        
Common Class S | 2030 Notes                
Investment Company, Financial Highlights [Roll Forward]                
Average market value per unit (in dollars per share) 1,001 996 1,001 996        
Common Class S | 2031 Notes                
Investment Company, Financial Highlights [Roll Forward]                
Average market value per unit (in dollars per share) $ 967 0 $ 967 0        
Common Class S | Other Short-Term Borrowings                
Investment Company, Financial Highlights [Roll Forward]                
Average market value per unit (in dollars per share)   $ 0   $ 0