Borrowings - Schedule of Maturities of Long-Term Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Sep. 30, 2021
Debt Instrument [Line Items]
Debt	$ 3,093,603	$ 2,569,228
Less Than 1 Year	0
1 – 3 Years	502,131
3 – 5 Years	1,637,316
More Than 5 Years	954,156
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	545,956
Less Than 1 Year	0
1 – 3 Years	0
3 – 5 Years	0
More Than 5 Years	545,956
JPM Credit Facility
Debt Instrument [Line Items]
Debt	692,592
Less Than 1 Year	0
1 – 3 Years	0
3 – 5 Years	692,592
More Than 5 Years	0
2024 Notes
Debt Instrument [Line Items]
Debt	502,131	400,000
Less Than 1 Year	0
1 – 3 Years	502,131
3 – 5 Years	0
More Than 5 Years	0
2026 Notes
Debt Instrument [Line Items]
Debt	597,930	400,000
Less Than 1 Year	0
1 – 3 Years	0
3 – 5 Years	597,930
More Than 5 Years	0
2027 Notes
Debt Instrument [Line Items]
Debt	346,794	$ 350,000
Less Than 1 Year	0
1 – 3 Years	0
3 – 5 Years	346,794
More Than 5 Years	$ 0