Borrowings - Schedule of Maturities of Long-Term Debt (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Sep. 30, 2024
Debt Instrument [Line Items]
Debt	$ 4,833,150	$ 4,624,791
Less Than 1 Year	0
1 – 3 Years	947,883
3 – 5 Years	2,521,267
More Than 5 Years	1,364,000
GBDC 3 2022 Debt Securitization
Debt Instrument [Line Items]
Debt	209,082
Less Than 1 Year	0
1 – 3 Years	0
3 – 5 Years	209,082
More Than 5 Years	0
2024 Debt Securitization
Debt Instrument [Line Items]
Debt	1,364,000
Less Than 1 Year	0
1 – 3 Years	0
3 – 5 Years	0
More Than 5 Years	1,364,000
JPM Credit Facility
Debt Instrument [Line Items]
Debt	1,110,054
Less Than 1 Year	0
1 – 3 Years	0
3 – 5 Years	1,110,054
More Than 5 Years	0
2026 Notes
Debt Instrument [Line Items]
Debt	599,258
Less Than 1 Year	0
1 – 3 Years	599,258
3 – 5 Years	0
More Than 5 Years	0
2027 Notes
Debt Instrument [Line Items]
Debt	348,625
Less Than 1 Year	0
1 – 3 Years	348,625
3 – 5 Years	0
More Than 5 Years	0
2028 Notes
Debt Instrument [Line Items]
Debt	453,645
Less Than 1 Year	0
1 – 3 Years	0
3 – 5 Years	453,645
More Than 5 Years	0
2029 Notes
Debt Instrument [Line Items]
Debt	748,486
Less Than 1 Year	0
1 – 3 Years	0
3 – 5 Years	748,486
More Than 5 Years	$ 0