Borrowings - Schedule of Maturities of Long-Term Debt (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Sep. 30, 2023
Debt Instrument [Line Items]
Debt	$ 4,337,697	$ 3,133,332
Less Than 1 Year	0
1 – 3 Years	946,936
3 – 5 Years	1,344,584
More Than 5 Years	2,046,177
2018 Debt Securitization
Debt Instrument [Line Items]
Debt	307,994
Less Than 1 Year	0
1 – 3 Years	0
3 – 5 Years	0
More Than 5 Years	307,994
GCIC 2018 Debt Securitization
Debt Instrument [Line Items]
Debt	380,017
Less Than 1 Year	0
1 – 3 Years	0
3 – 5 Years	0
More Than 5 Years	380,017
GBDC 3 2021 Debt Securitization(1)
Debt Instrument [Line Items]
Debt	297,950
Less Than 1 Year	0
1 – 3 Years	0
3 – 5 Years	0
More Than 5 Years	297,950
GBDC 3 2021 Debt Securitization(1) | GBDC 3
Debt Instrument [Line Items]
Discount on debt	50
GBDC 3 2022 Debt Securitization
Debt Instrument [Line Items]
Debt	251,587
Less Than 1 Year	0
1 – 3 Years	0
3 – 5 Years	0
More Than 5 Years	251,587
GBDC 3 2022-2 Debt Securitization
Debt Instrument [Line Items]
Debt	225,000
Less Than 1 Year	0
1 – 3 Years	0
3 – 5 Years	0
More Than 5 Years	225,000
DB Credit Facility
Debt Instrument [Line Items]
Debt	565,778
Less Than 1 Year	0
1 – 3 Years	0
3 – 5 Years	565,778
More Than 5 Years	0
JPM Credit Facility
Debt Instrument [Line Items]
Debt	333,718
Less Than 1 Year	0
1 – 3 Years	0
3 – 5 Years	333,718
More Than 5 Years	0
2026 Notes
Debt Instrument [Line Items]
Debt	598,860
Less Than 1 Year	0
1 – 3 Years	598,860
3 – 5 Years	0
More Than 5 Years	0
2027 Notes
Debt Instrument [Line Items]
Debt	348,076
Less Than 1 Year	0
1 – 3 Years	348,076
3 – 5 Years	0
More Than 5 Years	0
2028 Notes
Debt Instrument [Line Items]
Debt	445,088
Less Than 1 Year	0
1 – 3 Years	0
3 – 5 Years	445,088
More Than 5 Years	0
2029 Notes
Debt Instrument [Line Items]
Debt	583,629
Less Than 1 Year	0
1 – 3 Years	0
3 – 5 Years	0
More Than 5 Years	$ 583,629