v3.26.1
Borrowings - Schedule of Debt Securitization (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Debt Instrument [Line Items]        
Net (gain)/loss related to the fair value hedge $ (46) $ (155) $ 125 $ 4,541
Amortization of debt issuance costs 2,724 2,413 7,903 7,537
Cash paid for interest expense [1]     171,335 199,922
Average outstanding balance     4,763,136 4,867,651
2024 Debt Securitization        
Debt Instrument [Line Items]        
Stated interest expense 18,127 20,196 55,570 50,637
Amortization of debt issuance costs 232 232 696 573
Total interest and other debt financing expenses 18,359 20,428 56,266 51,210
Cash paid for interest expense $ 17,908 $ 34,891 $ 57,344 $ 34,891
Annualized average stated interest rate 5.30% 5.90% 5.40% 6.00%
Average outstanding balance $ 1,364,000 $ 1,364,000 $ 1,364,000 $ 1,124,176
2018 Debt Securitization        
Debt Instrument [Line Items]        
Stated interest expense 0 0 0 1,839
Amortization of debt issuance costs 0 0 0 0
Total interest and other debt financing expenses 0 0 0 1,839
Cash paid for interest expense 0 0 0 $ 5,152
Annualized average stated interest rate       7.10%
Average outstanding balance 0 0 0 $ 34,603
GCIC 2018 Debt Securitization        
Debt Instrument [Line Items]        
Stated interest expense 0 0 0 1,682
Amortization of debt issuance costs 0 0 0 0
Total interest and other debt financing expenses 0 0 0 1,682
Cash paid for interest expense 0 0 0 $ 4,861
Annualized average stated interest rate       6.10%
Average outstanding balance 0 0 0 $ 36,904
GBDC 3 2021 Debt Securitization        
Debt Instrument [Line Items]        
Stated interest expense 0 0 0 2,694
Amortization of debt issuance costs 0 0 0 0
Total interest and other debt financing expenses 0 0 0 2,694
Cash paid for interest expense 0 0 0 $ 7,355
Annualized average stated interest rate       6.90%
Average outstanding balance 0 0 0 $ 52,396
GBDC 3 2022 Debt Securitization        
Debt Instrument [Line Items]        
Stated interest expense 0 2,951 0 10,399
Amortization of debt issuance costs 0 0 0 0
Total interest and other debt financing expenses 0 2,951 0 10,399
Cash paid for interest expense 0 $ 3,289 0 $ 11,581
Annualized average stated interest rate   6.40%   6.60%
Average outstanding balance 0 $ 186,024 0 $ 211,325
GBDC 3 2022-2 Debt Securitization        
Debt Instrument [Line Items]        
Stated interest expense 0 0 0 3,504
Amortization of debt issuance costs 0 0 0 0
Total interest and other debt financing expenses 0 0 0 3,504
Cash paid for interest expense 0 0 0 $ 7,197
Annualized average stated interest rate       7.50%
Average outstanding balance 0 0 0 $ 62,637
GBDC 3 DB Credit Facility        
Debt Instrument [Line Items]        
Stated interest expense 0 0 0 3,820
Facility fees 0 0 0 244
Total interest and other debt financing expenses 0 0 0 4,064
Cash paid for interest expense 0 0 0 $ 11,680
Annualized average stated interest rate       6.80%
Average outstanding balance 0 0 0 $ 75,658
JPM Credit Facility        
Debt Instrument [Line Items]        
Stated interest expense 7,843 17,811 32,628 50,909
Facility fees 1,056 671 2,612 2,308
Amortization of debt issuance costs 939 888 2,771 3,039
Total interest and other debt financing expenses 9,838 19,370 38,011 56,256
Cash paid for interest expense $ 11,225 $ 18,162 $ 35,413 $ 55,617
Annualized average stated interest rate 4.50% 5.70% 4.70% 5.90%
Average outstanding balance $ 693,391 $ 1,262,514 $ 932,064 $ 1,154,567
2026 Notes        
Debt Instrument [Line Items]        
Stated interest expense 3,750 3,750 11,250 11,250
Accretion of discounts (premium) on notes issued 133 132 398 397
Amortization of debt issuance costs 372 372 1,116 1,161
Total interest and other debt financing expenses 4,255 4,254 12,764 12,808
Cash paid for interest expense $ 0 $ 0 $ 7,500 $ 7,500
Annualized average stated interest rate 2.50% 2.50% 2.50% 2.50%
Average outstanding balance $ 600,000 $ 600,000 $ 600,000 $ 600,000
2027 Notes        
Debt Instrument [Line Items]        
Stated interest expense 1,794 1,794 5,382 5,382
Accretion of discounts (premium) on notes issued 182 182 547 548
Amortization of debt issuance costs 203 203 609 635
Total interest and other debt financing expenses 2,179 2,179 6,538 6,565
Cash paid for interest expense $ 0 $ 0 $ 3,588 $ 3,588
Annualized average stated interest rate 2.10% 2.10% 2.10% 2.10%
Average outstanding balance $ 350,000 $ 350,000 $ 350,000 $ 350,000
2028 Notes        
Debt Instrument [Line Items]        
Stated interest expense 12,338 7,931 37,012 23,793
Net contractual interest rate swap expense (348) 197 (321) 1,050
Net (gain)/loss related to the fair value hedge (15) (64) 84 7,380
Accretion of discounts (premium) on notes issued (868) 241 (2,605) 722
Amortization of debt issuance costs 447 295 1,316 917
Total interest and other debt financing expenses 11,554 8,600 35,486 33,862
Cash paid for interest expense $ 13,091 $ 7,930 $ 40,985 $ 24,831
Annualized average stated interest rate 6.90% 7.20% 7.00% 7.40%
Average outstanding balance $ 700,000 $ 450,000 $ 700,000 $ 450,000
2029 Notes        
Debt Instrument [Line Items]        
Stated interest expense 11,250 11,250 33,750 32,200
Net contractual interest rate swap expense (190) 1,064 301 3,957
Net (gain)/loss related to the fair value hedge (31) (92) 41 (2,840)
Accretion of discounts (premium) on notes issued 272 272 816 841
Amortization of debt issuance costs 432 423 1,296 1,212
Total interest and other debt financing expenses 11,733 12,917 36,204 35,370
Cash paid for interest expense $ 2,131 $ 2,368 $ 26,414 $ 25,669
Annualized average stated interest rate 5.90% 6.60% 6.10% 6.80%
Average outstanding balance $ 750,000 $ 750,000 $ 750,000 $ 715,385
2031 Notes        
Debt Instrument [Line Items]        
Stated interest expense 2,951 0 2,951 0
Net contractual interest rate swap expense (126) 0 (126) 0
Accretion of discounts (premium) on notes issued 104 0 104 0
Amortization of debt issuance costs 99 0 99 0
Total interest and other debt financing expenses 3,028 0 3,028 0
Cash paid for interest expense $ 0 0 $ 0 0
Annualized average stated interest rate 5.90%   5.90%  
Average outstanding balance $ 192,308 0 $ 64,103 0
Adviser Revolver        
Debt Instrument [Line Items]        
Stated interest expense 46 0 79 0
Cash paid for interest expense $ 20 0 $ 91 0
Annualized average stated interest rate 3.50%   3.60%  
Average outstanding balance $ 5,266 $ 0 $ 2,969 $ 0
[1] Includes cash paid or received for contractual interest rate swaps, which may have different contractual settlement dates among the (i) fixed interest leg, (ii) floating interest leg and (iii) the debt instrument.