v3.25.2
Borrowings - Schedule of Debt Securitization (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Jun. 30, 2025
Jun. 30, 2024
Jun. 30, 2025
Jun. 30, 2024
Debt Instrument [Line Items]        
Net (gain)/loss related to the fair value hedge $ (155) $ 5,763 $ 4,541 $ 2,991
Accretion of discounts and amortization of premiums on notes issued     (5,524) 1,267
Amortization of debt issuance costs 2,413 2,214 7,537 6,869
Cash paid (received) for interest, other debt financing expenses, and facility fees [1]     199,922 110,623
Average outstanding balance     4,867,651 3,211,972
2024 Debt Securitization        
Debt Instrument [Line Items]        
Stated interest expense 20,196 0 50,637 0
Amortization of debt issuance costs 232 0 573 0
Total interest and other debt financing expenses 20,428 0 51,210 0
Cash paid (received) for interest, other debt financing expenses, and facility fees $ 34,891 0 $ 34,891 0
Annualized average stated interest rate 5.90%   6.00%  
Average outstanding balance $ 1,364,000 0 $ 1,124,176 0
2018 Debt Securitization        
Debt Instrument [Line Items]        
Stated interest expense 0 5,823 1,839 19,422
Amortization of debt issuance costs 0 0 0 2
Total interest and other debt financing expenses 0 5,823 1,839 19,424
Cash paid (received) for interest, other debt financing expenses, and facility fees 0 $ 6,354 $ 5,152 $ 20,766
Annualized average stated interest rate   7.40% 7.10% 7.40%
Average outstanding balance 0 $ 316,992 $ 34,603 $ 350,454
GCIC 2018 Debt Securitization        
Debt Instrument [Line Items]        
Stated interest expense 0 6,734 1,682 23,410
Accretion of discounts on notes issued 0 0 0 0
Amortization of debt issuance costs 0 0 0 0
Total interest and other debt financing expenses 0 6,734 1,682 23,410
Cash paid (received) for interest, other debt financing expenses, and facility fees 0 $ 7,928 $ 4,861 $ 25,531
Annualized average stated interest rate   6.80% 6.10% 6.80%
Average outstanding balance 0 $ 400,163 $ 36,904 $ 456,622
GBDC 3 2021 Debt Securitization        
Debt Instrument [Line Items]        
Stated interest expense 0 1,680 2,694 1,680
Amortization of debt issuance costs 0 0 0 0
Total interest and other debt financing expenses 0 1,680 2,694 1,680
Cash paid (received) for interest, other debt financing expenses, and facility fees 0 $ 0 $ 7,355 $ 0
Annualized average stated interest rate   7.40% 6.90% 7.40%
Average outstanding balance 0 $ 91,692 $ 52,396 $ 30,452
GBDC 3 2022 Debt Securitization        
Debt Instrument [Line Items]        
Stated interest expense 2,951 1,437 10,399 1,437
Amortization of debt issuance costs 0 0 0 0
Total interest and other debt financing expenses 2,951 1,437 10,399 1,437
Cash paid (received) for interest, other debt financing expenses, and facility fees $ 3,289 $ 0 $ 11,581 $ 0
Annualized average stated interest rate 6.40% 7.50% 6.60% 7.50%
Average outstanding balance $ 186,024 $ 77,411 $ 211,325 $ 25,710
GBDC 3 2022-2 Debt Securitization        
Debt Instrument [Line Items]        
Stated interest expense 0 1,387 3,504 1,387
Amortization of debt issuance costs 0 0 0 0
Total interest and other debt financing expenses 0 1,387 3,504 1,387
Cash paid (received) for interest, other debt financing expenses, and facility fees 0 $ 0 $ 7,197 $ 0
Annualized average stated interest rate   8.10% 7.50% 8.10%
Average outstanding balance 0 $ 69,231 $ 62,637 $ 22,993
GBDC 3 DB Credit Facility        
Debt Instrument [Line Items]        
Stated interest expense 0 3,076 3,820 3,076
Facility fees 0 154 244 154
Accretion of discounts and amortization of premiums on notes issued 0 0 0 0
Total interest and other debt financing expenses 0 3,230 4,064 3,230
Cash paid (received) for interest, other debt financing expenses, and facility fees 0 $ 0 $ 11,680 $ 0
Annualized average stated interest rate   7.60% 6.80% 7.60%
Average outstanding balance 0 $ 163,225 $ 75,658 $ 54,210
JPM Credit Facility        
Debt Instrument [Line Items]        
Stated interest expense 17,811 3,423 50,909 15,898
Facility fees 671 1,125 2,308 3,358
Amortization of debt issuance costs 888 848 3,039 2,491
Total interest and other debt financing expenses 19,370 5,396 56,256 21,747
Cash paid (received) for interest, other debt financing expenses, and facility fees $ 18,162 $ 4,083 $ 55,617 $ 18,818
Annualized average stated interest rate 5.70% 7.10% 5.90% 7.10%
Average outstanding balance $ 1,262,514 $ 194,775 $ 1,154,567 $ 300,911
2024 Notes        
Debt Instrument [Line Items]        
Stated interest expense 0 328 0 8,766
Accretion of discounts and amortization of premiums on notes issued 0 (53) 0 (747)
Amortization of debt issuance costs 0 84 0 1,204
Total interest and other debt financing expenses 0 359 0 9,223
Cash paid (received) for interest, other debt financing expenses, and facility fees 0 $ 8,109 0 $ 16,547
Annualized average stated interest rate   3.40%   3.40%
Average outstanding balance 0 $ 38,462 0 $ 346,715
2026 Notes        
Debt Instrument [Line Items]        
Stated interest expense 3,750 3,750 11,250 11,250
Accretion of discounts on notes issued 132 132 397 398
Amortization of debt issuance costs 372 420 1,161 1,269
Total interest and other debt financing expenses 4,254 4,302 12,808 12,917
Cash paid (received) for interest, other debt financing expenses, and facility fees $ 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 on notes issued 182 182 548 549
Amortization of debt issuance costs 203 231 635 698
Total interest and other debt financing expenses 2,179 2,207 6,565 6,629
Cash paid (received) for interest, other debt financing expenses, and facility fees $ 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 7,931 7,931 23,793 18,153
Net contractual interest rate swap expense 197 1,279 1,050 2,368
Net (gain)/loss related to the fair value hedge (64) 3,906 7,380 (279)
Accretion of discounts on notes issued 241 241 722 552
Amortization of debt issuance costs 295 299 917 661
Total interest and other debt financing expenses 8,600 13,656 33,862 21,455
Cash paid (received) for interest, other debt financing expenses, and facility fees $ 7,930 $ 12,944 $ 24,831 $ 17,873
Annualized average stated interest rate 7.20% 8.20% 7.40% 8.00%
Average outstanding balance $ 450,000 $ 450,000 $ 450,000 $ 343,248
2029 Notes        
Debt Instrument [Line Items]        
Stated interest expense 11,250 9,000 32,200 15,000
Net contractual interest rate swap expense 1,064 2,405 3,957 3,923
Net (gain)/loss related to the fair value hedge (92) 1,857 (2,840) 3,270
Accretion of discounts on notes issued 272 309 841 513
Amortization of debt issuance costs 423 333 1,212 543
Total interest and other debt financing expenses 12,917 13,904 35,370 23,249
Cash paid (received) for interest, other debt financing expenses, and facility fees $ 2,368 $ 0 $ 25,669 $ 0
Annualized average stated interest rate 6.60% 7.60% 6.80% 7.60%
Average outstanding balance $ 750,000 $ 600,000 $ 715,385 $ 330,657
Adviser Revolver        
Debt Instrument [Line Items]        
Stated interest expense 0 0 0 0
Cash paid (received) for interest, other debt financing expenses, and facility fees 0 0 0 0
Average outstanding balance $ 0 $ 0 $ 0 $ 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.