v3.26.1
Financial Highlights (Details)
$ / shares in Units, $ in Thousands
 3 Months Ended 9 Months Ended
Jun. 30, 2026
USD ($)
$ / shares
shares
Jun. 30, 2025
USD ($)
$ / shares
shares
Jun. 30, 2026
USD ($)
$ / shares
shares
Jun. 30, 2025
USD ($)
$ / shares
shares
Mar. 31, 2026
USD ($)
Sep. 30, 2025
USD ($)
shares
Mar. 31, 2025
USD ($)
Sep. 30, 2024
USD ($)
Feb. 06, 2019
Feb. 05, 2019
Feb. 04, 2019
Investment Company, Financial Highlights [Roll Forward]                      
Net asset value at beginning of period (in dollars per share)     $ 14.97 $ 15.19              
Net increase in net assets as a result of repurchases of shares (in dollars per share)     0.04 0.01              
Distributions declared:                      
From net investment income - after tax (in dollars per share)     (1.05) (1.26)              
Net investment income - after tax (in dollars per share)     1.02 1.12              
Net realized gain (loss) on investment transactions (in dollars per share)     (0.15) (0.15)              
Net realized gain (loss) on extinguishment of debt (in dollars per share)     0                
Net change in unrealized appreciation (depreciation) on investment transactions (in dollars per share)     (0.58) 0.09              
Net asset value at end of period (in dollars per share) $ 14.25 $ 15.00 14.25 15.00              
Per share market value at end of period (in dollars per share) $ 12.88 $ 14.65 $ 12.88 $ 14.65              
Total return based on market value     1.61% 5.10%              
Number of common shares outstanding (in shares) | shares 260,033,889 266,376,416 260,033,889 266,376,416   266,008,083          
Ratio of net investment income - after tax to average net assets     9.22% 9.84%              
Ratio of total expenses to average net assets     10.51% 11.37%              
Ratio of incentive fees to average net assets     1.25% 1.36%              
Ratio of income and excise taxes to average net assets     0.00% (0.01%)              
Ratio of net expenses (without incentive fees) to average net assets     9.26% 10.01%              
Total return based on average net asset value     1.95% 6.96%              
Total return based on average net asset value - annualized     2.61% 9.30%              
Net assets at end of period | $ $ 3,704,869 $ 3,995,329 $ 3,704,869 $ 3,995,329 $ 3,748,120 $ 3,982,567 $ 4,043,498 $ 4,014,529      
Average debt outstanding | $     $ 4,763,136 $ 4,867,651              
Average debt outstanding per share (in dollars per share)     $ 18.32 $ 18.27              
Portfolio turnover     5.78% 17.80%              
Asset coverage ratio 180.37% 176.75% 180.37% 176.75%              
Asset coverage ratio per unit (in dollars per share) $ 1,804 $ 1,768,000 $ 1,804 $ 1,768,000              
Minimum asset coverage ratio required                 1.50 1.50 2
GCIC 2018 Debt Securitization                      
Distributions declared:                      
Average debt outstanding | $ $ 0 $ 0 $ 0 $ 36,904              
GBDC 3 2021 Debt Securitization                      
Distributions declared:                      
Average debt outstanding | $ 0 0 0 52,396              
GBDC 3 2022 Debt Securitization                      
Distributions declared:                      
Average debt outstanding | $ 0 186,024 0 211,325              
GBDC 3 2022-2 Debt Securitization                      
Distributions declared:                      
Average debt outstanding | $ 0 0 0 62,637              
GBDC 3 DB Credit Facility                      
Distributions declared:                      
Average debt outstanding | $ 0 0 0 75,658              
2026 Notes                      
Distributions declared:                      
Average debt outstanding | $ $ 600,000 $ 600,000 $ 600,000 $ 600,000              
Average market value per unit (in dollars per share) $ 989 $ 960,000 $ 989 $ 960,000              
2027 Notes                      
Distributions declared:                      
Average debt outstanding | $ $ 350,000 $ 350,000 $ 350,000 $ 350,000              
Average market value per unit (in dollars per share) $ 973 $ 938,000 $ 973 $ 938,000              
2028 Notes                      
Distributions declared:                      
Average debt outstanding | $ $ 700,000 $ 450,000 $ 700,000 $ 450,000              
Average market value per unit (in dollars per share) $ 1,040 $ 1,046,000 $ 1,040 $ 1,046,000              
2029 Notes                      
Distributions declared:                      
Average debt outstanding | $ $ 750,000 $ 750,000 $ 750,000 $ 715,385              
Average market value per unit (in dollars per share) $ 1,010 $ 1,003,000 $ 1,010 $ 1,003,000              
2031 Notes                      
Distributions declared:                      
Average debt outstanding | $ $ 192,308 $ 0 $ 64,103 $ 0              
Average market value per unit (in dollars per share) $ 994   $ 994