Borrowings - Schedule of Maturities of Long-Term Debt (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Sep. 30, 2024
Debt Instrument [Line Items]
Debt	$ 5,154,001	$ 4,624,791
Less Than 1 Year	0
1 – 3 Years	948,198
3 – 5 Years	2,841,803
More Than 5 Years	1,364,000
GBDC 3 2022 Debt Securitization
Debt Instrument [Line Items]
Debt	179,529
Less Than 1 Year	0
1 – 3 Years	0
3 – 5 Years	179,529
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	1,450,188
Less Than 1 Year	0
1 – 3 Years	0
3 – 5 Years	1,450,188
More Than 5 Years	0
2026 Notes
Debt Instrument [Line Items]
Debt	599,391
Less Than 1 Year	0
1 – 3 Years	599,391
3 – 5 Years	0
More Than 5 Years	0
2027 Notes
Debt Instrument [Line Items]
Debt	348,807
Less Than 1 Year	0
1 – 3 Years	348,807
3 – 5 Years	0
More Than 5 Years	0
2028 Notes
Debt Instrument [Line Items]
Debt	456,834
Less Than 1 Year	0
1 – 3 Years	0
3 – 5 Years	456,834
More Than 5 Years	0
2029 Notes
Debt Instrument [Line Items]
Debt	755,252
Less Than 1 Year	0
1 – 3 Years	0
3 – 5 Years	755,252
More Than 5 Years	$ 0