Borrowings - Schedule of Maturities of Long-Term Debt (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Sep. 30, 2022
Debt Instrument [Line Items]
Debt	$ 3,116,857	$ 3,093,603
Less Than 1 Year	0
1 – 3 Years	538,941
3 – 5 Years	1,623,216
More Than 5 Years	954,700
2018 Debt Securitization
Debt Instrument [Line Items]
Debt	408,200
Less Than 1 Year	0
1 – 3 Years	0
3 – 5 Years	0
More Than 5 Years	408,200
2018 GCIC Debt Securitization
Debt Instrument [Line Items]
Debt	546,500
Less Than 1 Year	0
1 – 3 Years	0
3 – 5 Years	0
More Than 5 Years	546,500
JPM Credit Facility
Debt Instrument [Line Items]
Debt	715,362
Less Than 1 Year	0
1 – 3 Years	37,500
3 – 5 Years	677,862
More Than 5 Years	0
2024 Notes
Debt Instrument [Line Items]
Debt	501,441	500,000
Less Than 1 Year	0
1 – 3 Years	501,441
3 – 5 Years	0
More Than 5 Years	0
2026 Notes
Debt Instrument [Line Items]
Debt	598,195	600,000
Less Than 1 Year	0
1 – 3 Years	0
3 – 5 Years	598,195
More Than 5 Years	0
2027 Notes
Debt Instrument [Line Items]
Debt	347,159	$ 350,000
Less Than 1 Year	0
1 – 3 Years	0
3 – 5 Years	347,159
More Than 5 Years	$ 0