Borrowings - Interest Rate and Interest Differential Analysis (Details) - USD ($) $ in Thousands	6 Months Ended
	Mar. 31, 2023	Sep. 30, 2022
Debt Instrument [Line Items]
Debt	$ 3,116,857	$ 3,093,603
Class A 2018 Notes | Senior Notes
Debt Instrument [Line Items]
Debt	$ 327,000
Class A 2018 Notes | Senior Notes | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	1.48%
Class B 2018 Notes | Senior Notes
Debt Instrument [Line Items]
Debt	$ 61,200
Class B 2018 Notes | Senior Notes | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	2.10%
Class C-1 2018 Notes | Senior Notes
Debt Instrument [Line Items]
Debt	$ 20,000
Class C-1 2018 Notes | Senior Notes | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	2.80%
Class A-1 GCIC 2018 Notes | Senior Notes
Debt Instrument [Line Items]
Debt	$ 490,000
Class A-1 GCIC 2018 Notes | Senior Notes | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	1.48%
Class A-2-R GCIC 2018 Notes | Senior Notes
Debt Instrument [Line Items]
Debt	$ 38,500
Debt instrument, basis spread on variable rate	2.50%
Class B-1 GCIC 2018 Notes | Senior Notes
Debt Instrument [Line Items]
Debt	$ 18,000
Class B-1 GCIC 2018 Notes | Senior Notes | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	2.25%