v3.25.2
Financial Highlights (Details)
$ / shares in Units, $ in Thousands
 3 Months Ended 9 Months Ended
Jun. 30, 2025
USD ($)
$ / shares
shares
Jun. 30, 2024
USD ($)
$ / shares
shares
Jun. 30, 2025
USD ($)
$ / shares
shares
Jun. 30, 2024
USD ($)
$ / shares
shares
Mar. 31, 2025
USD ($)
Sep. 30, 2024
USD ($)
shares
Mar. 31, 2024
USD ($)
Sep. 30, 2023
USD ($)
Feb. 06, 2019
Feb. 05, 2019
Investment Company, Financial Highlights [Roll Forward]                    
Net asset value at beginning of period (in dollars per share)     $ 15.19 $ 15.02            
Net increase (decrease) in net assets as a result of issuance of shares (in dollars per share)     0 0.51            
Net increase in net assets as a result of repurchases of shares (in dollars per share)     0.01 0            
Distributions declared:                    
From net investment income - after tax (in dollars per share)     (1.26) (1.40)            
Net investment income - after tax (in dollars per share)     1.12 1.46            
Net realized gain (loss) on investment transactions (in dollars per share)     (0.15) (0.28)            
Net realized gain (loss) on extinguishment of debt (in dollars per share)     0.00 0            
Net change in unrealized appreciation (depreciation) on investment transactions (in dollars per share)     0.09 0.01            
Net asset value at end of period (in dollars per share) $ 15.00 $ 15.32 15.00 15.32            
Per share market value at end of period (in dollars per share) $ 14.65 $ 15.71 $ 14.65 $ 15.71            
Total return based on market value     5.10% 17.41%            
Number of common shares outstanding (in shares) | shares 266,376,416 264,609,056 266,376,416 264,609,056   264,277,128        
Ratio of net investment income - after tax to average net assets     9.84% 12.92%            
Ratio of total expenses to average net assets     11.37% 11.92%            
Ratio of incentive fee waiver to average net assets     0.00% (0.72%)            
Ratio of incentive fees to average net assets     1.36% 2.07%            
Ratio of income and excise taxes to average net assets     (0.01%) 0.04%            
Ratio of net expenses (without incentive fees) to average net assets     10.01% 9.85%            
Total return based on average net asset value     6.96% 6.57%            
Total return based on average net asset value - annualized     9.30% 8.78%            
Net assets at end of period | $ $ 3,995,329 $ 4,053,643 $ 3,995,329 $ 4,053,643 $ 4,043,498 $ 4,014,529 $ 2,593,640 $ 2,547,878    
Average debt outstanding | $     $ 4,867,651 $ 3,211,972            
Average debt outstanding per share (in dollars per share)     $ 18.27 $ 12.14            
Portfolio Turnover     17.80% 13.11%            
Asset coverage ratio 176.75% 192.15% 176.75% 192.15%            
Asset coverage ratio per unit (in dollars per share) $ 1,768 $ 1,922 $ 1,768 $ 1,922            
Minimum asset coverage ratio required                 1.50 1.50
2026 Notes                    
Distributions declared:                    
Average debt outstanding | $ $ 600,000 $ 600,000 $ 600,000 $ 600,000            
Average market value per unit (in dollars per share) $ 960 $ 909 $ 960 $ 909            
2027 Notes                    
Distributions declared:                    
Average debt outstanding | $ $ 350,000 $ 350,000 $ 350,000 $ 350,000            
Average market value per unit (in dollars per share) $ 938 $ 882 $ 938 $ 882            
2028 Notes                    
Distributions declared:                    
Average debt outstanding | $ $ 450,000 $ 450,000 $ 450,000 $ 343,248            
Average market value per unit (in dollars per share) $ 1,046 $ 1,027 $ 1,046 $ 1,027            
2029 Notes                    
Distributions declared:                    
Average debt outstanding | $ $ 750,000 $ 600,000 $ 715,385 $ 330,657            
Average market value per unit (in dollars per share) $ 1,003 $ 980 $ 1,003 $ 980