Borrowings - Schedule of Maturities of Long-Term Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Sep. 30, 2024
Debt Instrument [Line Items]
Debt	$ 4,919,258	$ 4,624,791
Less Than 1 Year	0
1 – 3 Years	947,572
3 – 5 Years	2,375,173
More Than 5 Years	1,596,513
GBDC 3 2022 Debt Securitization
Debt Instrument [Line Items]
Debt	232,513
Less Than 1 Year	0
1 – 3 Years	0
3 – 5 Years	0
More Than 5 Years	232,513
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	1,191,300
Less Than 1 Year	0
1 – 3 Years	0
3 – 5 Years	1,191,300
More Than 5 Years	0
2026 Notes
Debt Instrument [Line Items]
Debt	599,127
Less Than 1 Year	0
1 – 3 Years	599,127
3 – 5 Years	0
More Than 5 Years	0
2027 Notes
Debt Instrument [Line Items]
Debt	348,445
Less Than 1 Year	0
1 – 3 Years	348,445
3 – 5 Years	0
More Than 5 Years	0
2028 Notes
Debt Instrument [Line Items]
Debt	447,495
Less Than 1 Year	0
1 – 3 Years	0
3 – 5 Years	447,495
More Than 5 Years	0
2029 Notes
Debt Instrument [Line Items]
Debt	736,378
Less Than 1 Year	0
1 – 3 Years	0
3 – 5 Years	736,378
More Than 5 Years	$ 0