v3.25.0.1
Borrowings - Schedule of Debt Securitization (Details) - USD ($)
$ in Thousands
 3 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Debt Instrument [Line Items]    
Net (gain)/loss related to the fair value hedge $ 4,662 $ (301)
Accretion of discounts and amortization of premiums on notes issued (569) (3,673)
Amortization of debt issuance costs 2,645 2,109
Cash paid for interest, other debt financing expenses, and facility fees 69,118 37,576
Average outstanding balance 4,770,127 3,078,186
2024 Debt Securitization    
Debt Instrument [Line Items]    
Stated interest expense 10,212  
Amortization of debt issuance costs 112  
Total interest and other debt financing expenses 10,324  
Cash paid for interest, other debt financing expenses, and facility fees $ 0  
Annualized average stated interest rate 6.20%  
Average outstanding balance $ 652,348  
2018 Debt Securitization    
Debt Instrument [Line Items]    
Stated interest expense 1,839 7,096
Amortization of debt issuance costs 0 2
Total interest and other debt financing expenses 1,839 7,098
Cash paid for interest, other debt financing expenses, and facility fees $ 5,152 $ 7,185
Annualized average stated interest rate 7.10% 7.40%
Average outstanding balance $ 102,681 $ 379,974
GCIC 2018 Debt Securitization    
Debt Instrument [Line Items]    
Stated interest expense 1,682 8,627
Accretion of discounts on notes issued 0 0
Amortization of debt issuance costs 0 0
Total interest and other debt financing expenses 1,682 8,627
Cash paid for interest, other debt financing expenses, and facility fees $ 4,861 $ 8,855
Annualized average stated interest rate 6.10% 6.90%
Average outstanding balance $ 109,509 $ 496,297
GBDC 3 2021 Debt Securitization    
Debt Instrument [Line Items]    
Stated interest expense 2,694 0
Amortization of debt issuance costs 0 0
Total interest and other debt financing expenses 2,694 0
Cash paid for interest, other debt financing expenses, and facility fees $ 7,355 0
Annualized average stated interest rate 6.90%  
Average outstanding balance $ 155,478 0
GBDC 3 2022 Debt Securitization    
Debt Instrument [Line Items]    
Stated interest expense 3,504 0
Amortization of debt issuance costs 0 0
Total interest and other debt financing expenses 3,504 0
Cash paid for interest, other debt financing expenses, and facility fees $ 4,360 0
Annualized average stated interest rate 7.50%  
Average outstanding balance $ 185,870 0
GBDC 3 2022-2 Debt Securitization    
Debt Instrument [Line Items]    
Stated interest expense 4,035 0
Amortization of debt issuance costs 0 0
Total interest and other debt financing expenses 4,035 0
Cash paid for interest, other debt financing expenses, and facility fees $ 7,197 0
Annualized average stated interest rate 6.90%  
Average outstanding balance $ 233,453 0
GBDC 3 DB Credit Facility    
Debt Instrument [Line Items]    
Stated interest expense 3,820 0
Facility fees 244 0
Accretion of discounts and amortization of premiums on notes issued 0 0
Total interest and other debt financing expenses 4,064 0
Cash paid for interest, other debt financing expenses, and facility fees $ 11,680 0
Annualized average stated interest rate 6.90%  
Average outstanding balance $ 220,086 0
JPM Credit Facility    
Debt Instrument [Line Items]    
Stated interest expense 17,045 10,836
Facility fees 788 822
Amortization of debt issuance costs 1,231 822
Total interest and other debt financing expenses 19,064 12,480
Cash paid for interest, other debt financing expenses, and facility fees $ 20,021 $ 13,098
Annualized average stated interest rate 6.40% 7.00%
Average outstanding balance $ 1,063,419 $ 619,850
2024 Notes    
Debt Instrument [Line Items]    
Stated interest expense 0 4,219
Accretion of discounts and amortization of premiums on notes issued 0 (349)
Amortization of debt issuance costs 0 534
Total interest and other debt financing expenses 0 4,404
Cash paid for interest, other debt financing expenses, and facility fees 0 $ 8,438
Annualized average stated interest rate   3.40%
Average outstanding balance 0 $ 500,000
2026 Notes    
Debt Instrument [Line Items]    
Stated interest expense 3,750 3,750
Accretion of discounts on notes issued 134 134
Amortization of debt issuance costs 403 416
Total interest and other debt financing expenses 4,287 4,300
Cash paid for interest, other debt financing expenses, and facility fees $ 0 $ 0
Annualized average stated interest rate 2.50% 2.50%
Average outstanding balance $ 600,000 $ 600,000
2027 Notes    
Debt Instrument [Line Items]    
Stated interest expense 1,794 1,794
Accretion of discounts on notes issued 185 185
Amortization of debt issuance costs 221 229
Total interest and other debt financing expenses 2,200 2,208
Cash paid for interest, other debt financing expenses, and facility fees $ 0 $ 0
Annualized average stated interest rate 2.10% 2.10%
Average outstanding balance $ 350,000 $ 350,000
2028 Notes    
Debt Instrument [Line Items]    
Stated interest expense 7,931 2,291
Net contractual interest rate swap expense 738 276
Net (gain)/loss related to the fair value hedge 7,409 (301)
Accretion of discounts on notes issued 243 71
Amortization of debt issuance costs 318 106
Total interest and other debt financing expenses 16,639 2,443
Cash paid for interest, other debt financing expenses, and facility fees $ 8,492 $ 0
Annualized average stated interest rate 7.60% 6.90%
Average outstanding balance $ 450,000 $ 132,065
2029 Notes    
Debt Instrument [Line Items]    
Stated interest expense 9,700 0
Net contractual interest rate swap expense 1,697 0
Net (gain)/loss related to the fair value hedge (2,747) 0
Accretion of discounts on notes issued 301 0
Amortization of debt issuance costs 360 0
Total interest and other debt financing expenses 9,311 0
Cash paid for interest, other debt financing expenses, and facility fees $ 0 0
Annualized average stated interest rate 7.00%  
Average outstanding balance $ 647,283 0
Adviser Revolver    
Debt Instrument [Line Items]    
Stated interest expense 0 0
Cash paid for interest, other debt financing expenses, and facility fees 0 0
Average outstanding balance $ 0 $ 0