Borrowings - Schedule of Debt Securitization (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Debt Instrument [Line Items]
Accretion of discounts and amortization of premiums on notes issued			$ 1,267	$ 454
Amortization of debt issuance costs	$ 2,214	$ 1,937	6,869	5,362
Cash paid for interest expense			110,623	98,381
Average outstanding balance			3,211,972	3,071,718
Interest rate swaps
Debt Instrument [Line Items]
Net (gain)/loss related to the fair value hedge	5,763		2,991
2018 Debt Securitization
Debt Instrument [Line Items]
Stated interest expense	5,823	6,947	19,422	19,207
Amortization of debt issuance costs	0	9	2	152
Total interest and other debt financing expenses	5,823	6,956	19,424	19,359
Cash paid for interest expense	$ 6,354	$ 6,577	$ 20,766	$ 17,246
Annualized average stated interest rate	7.40%	6.90%	7.40%	6.30%
Average outstanding balance	$ 316,992	$ 404,387	$ 350,454	$ 406,929
GCIC 2018 Debt Securitization
Debt Instrument [Line Items]
Stated interest expense	6,734	8,670	23,410	24,068
Accretion of discounts on notes issued	0	0	0	544
Amortization of debt issuance costs	0	0	0	21
Total interest and other debt financing expenses	6,734	8,670	23,410	24,633
Cash paid for interest expense	$ 7,928	$ 8,261	$ 25,531	$ 21,681
Annualized average stated interest rate	6.80%	6.40%	6.80%	5.90%
Average outstanding balance	$ 400,163	$ 539,674	$ 456,622	$ 544,225
GBDC 3 2021 Debt Securitization(1)
Debt Instrument [Line Items]
Stated interest expense	1,680		1,680
Amortization of debt issuance costs	0		0
Total interest and other debt financing expenses	1,680		1,680
Cash paid for interest expense	$ 0		$ 0
Annualized average stated interest rate	7.40%		7.40%
Average outstanding balance	$ 91,692		$ 30,452
GBDC 3 2022 Debt Securitization
Debt Instrument [Line Items]
Stated interest expense	1,437		1,437
Amortization of debt issuance costs	0		0
Total interest and other debt financing expenses	1,437		1,437
Cash paid for interest expense	$ 0		$ 0
Annualized average stated interest rate	7.50%		7.50%
Average outstanding balance	$ 77,411		$ 25,710
GBDC 3 2022-2 Debt Securitization
Debt Instrument [Line Items]
Stated interest expense	1,387		1,387
Amortization of debt issuance costs	0		0
Total interest and other debt financing expenses	1,387		1,387
Cash paid for interest expense	$ 0		$ 0
Annualized average stated interest rate	8.10%		8.10%
Average outstanding balance	$ 69,231		$ 22,993
DB Credit Facility
Debt Instrument [Line Items]
Stated interest expense	3,076		3,076
Facility fees	154		154
Amortization of debt issuance costs	0		0
Total interest and other debt financing expenses	3,230		3,230
Cash paid for interest expense	$ 0		$ 0
Annualized average stated interest rate	7.60%		7.60%
Average outstanding balance	$ 163,225		$ 54,210
JPM Credit Facility
Debt Instrument [Line Items]
Stated interest expense	3,423	11,195	15,898	29,463
Facility fees	1,125	1,005	3,358	2,145
Amortization of debt issuance costs	848	836	2,491	1,948
Total interest and other debt financing expenses	5,396	13,036	21,747	33,556
Cash paid for interest expense	$ 4,083	$ 11,704	$ 18,818	$ 31,491
Annualized average stated interest rate	7.10%	6.40%	7.10%	5.90%
Average outstanding balance	$ 194,775	$ 703,448	$ 300,911	$ 670,564
2024 Notes
Debt Instrument [Line Items]
Stated interest expense	328	4,219	8,766	12,657
Accretion of discounts and amortization of premiums on notes issued	(53)	(345)	(747)	(1,035)
Amortization of debt issuance costs	84	495	1,204	1,485
Total interest and other debt financing expenses	359	4,369	9,223	13,107
Cash paid for interest expense	$ 8,109	$ 8,438	$ 16,547	$ 16,876
Annualized average stated interest rate	3.40%	3.40%	3.40%	3.40%
Average outstanding balance	$ 38,462	$ 500,000	$ 346,715	$ 500,000
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	398	397
Amortization of debt issuance costs	420	372	1,269	1,117
Total interest and other debt financing expenses	4,302	4,254	12,917	12,764
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 on notes issued	182	183	549	548
Amortization of debt issuance costs	231	225	698	639
Total interest and other debt financing expenses	2,207	2,202	6,629	6,569
Cash paid for interest expense	$ 0	$ 0	$ 3,588	$ 3,587
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		18,153
Accretion of discounts on notes issued	241		552
Amortization of debt issuance costs	299		661
Total interest and other debt financing expenses	13,656		21,455
Cash paid for interest expense	$ 12,944		$ 17,873
Annualized average stated interest rate	8.20%		8.00%
Average outstanding balance	$ 450,000		$ 343,248
2028 Notes | Interest rate swaps
Debt Instrument [Line Items]
Net contractual interest rate swap expense	1,279		2,368
Net (gain)/loss related to the fair value hedge	(3,906)		279
2029 Notes
Debt Instrument [Line Items]
Stated interest expense	9,000		15,000
Accretion of discounts on notes issued	309		513
Amortization of debt issuance costs	333		543
Total interest and other debt financing expenses	13,904		23,249
Cash paid for interest expense	$ 0		$ 0
Annualized average stated interest rate	7.60%		7.60%
Average outstanding balance	$ 600,000		$ 330,657
2029 Notes | Interest rate swaps
Debt Instrument [Line Items]
Net contractual interest rate swap expense	2,405		3,923
Net (gain)/loss related to the fair value hedge	1,857		3,270
Adviser Revolver
Debt Instrument [Line Items]
Stated interest expense	0	0	0	0
Cash paid for interest expense	$ 0	$ 0	$ 0	$ 0
Annualized average stated interest rate	0.00%	0.00%	0.00%	0.00%
Average outstanding balance	$ 0	$ 0	$ 0	$ 0