Borrowings - JPM Credit Facility Narrative (Details) - USD ($) $ in Thousands				6 Months Ended
	Mar. 17, 2023	Sep. 13, 2022	Nov. 19, 2021	Mar. 31, 2023	Sep. 30, 2022
Debt Instrument [Line Items]
Debt				$ 3,116,857	$ 3,093,603
JPM Credit Facility
Debt Instrument [Line Items]
Debt				715,362
Revolving Credit Facility | JPM Credit Facility | Line of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity				1,487,500
Line of credit facility, increase (decrease), net				$ 512,500
Line of credit facility, commitment fee percentage				0.375%
Debt instrument, amortization period				1 year
Debt				$ 715,362	$ 692,592
Revolving Credit Facility | JPM Credit Facility | Line of Credit | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Debt Instrument [Line Items]
Line of credit facility, interest rate during period	0.10%
Revolving Credit Facility | JPM Credit Facility | Line of Credit | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate | Minimum
Debt Instrument [Line Items]
Line of credit facility, interest rate during period		0.11448%
Revolving Credit Facility | JPM Credit Facility | Line of Credit | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate | Maximum
Debt Instrument [Line Items]
Line of credit facility, interest rate during period		0.42826%
Revolving Credit Facility | JPM Credit Facility | Line of Credit | Base Rate Option - Equal To Or Greater Than
Debt Instrument [Line Items]
Line of credit facility, gross borrowing base, outstanding indebtedness percentage				160.00%
Revolving Credit Facility | JPM Credit Facility | Line of Credit | Base Rate Option - Equal To Or Greater Than | Overnight Bank Funding Rate
Debt Instrument [Line Items]
Line of credit facility, interest rate during period				50.00%
Revolving Credit Facility | JPM Credit Facility | Line of Credit | Base Rate Option - Equal To Or Greater Than | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Line of credit facility, interest rate during period				1.00%
Revolving Credit Facility | JPM Credit Facility | Line of Credit | Base Rate Option - Equal To Or Greater Than | Alternative Base Rate
Debt Instrument [Line Items]
Line of credit facility, interest rate during period				0.75%
Revolving Credit Facility | JPM Credit Facility | Line of Credit | Base Rate Option - Less Than
Debt Instrument [Line Items]
Line of credit facility, gross borrowing base, combined debt percentage				160.00%
Revolving Credit Facility | JPM Credit Facility | Line of Credit | Base Rate Option - Less Than | Alternative Base Rate
Debt Instrument [Line Items]
Line of credit facility, interest rate during period				87.50%
Revolving Credit Facility | JPM Credit Facility | Line of Credit | Eurocurrency Option - Equal To Or Greater Than
Debt Instrument [Line Items]
Line of credit facility, gross borrowing base, combined debt percentage			160.00%	160.00%
Revolving Credit Facility | JPM Credit Facility | Line of Credit | Eurocurrency Option - Equal To Or Greater Than | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Line of credit facility, interest rate during period				1.75%
Revolving Credit Facility | JPM Credit Facility | Line of Credit | Eurocurrency Option - Equal To Or Greater Than | SONIA
Debt Instrument [Line Items]
Line of credit facility, interest rate during period			1.7826%
Revolving Credit Facility | JPM Credit Facility | Line of Credit | Eurocurrency Option - Equal To Or Greater Than | Swiss Average Overnight Rate
Debt Instrument [Line Items]
Line of credit facility, interest rate during period			1.6929%
Revolving Credit Facility | JPM Credit Facility | Line of Credit | Eurocurrency Option - Less Than
Debt Instrument [Line Items]
Line of credit facility, gross borrowing base, combined debt percentage			160.00%	160.00%
Revolving Credit Facility | JPM Credit Facility | Line of Credit | Eurocurrency Option - Less Than | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Line of credit facility, interest rate during period				1.875%
Revolving Credit Facility | JPM Credit Facility | Line of Credit | Eurocurrency Option - Less Than | SONIA
Debt Instrument [Line Items]
Line of credit facility, interest rate during period			1.9076%
Revolving Credit Facility | JPM Credit Facility | Line of Credit | Eurocurrency Option - Less Than | SARON
Debt Instrument [Line Items]
Line of credit facility, interest rate during period			1.8179%
Revolving Credit Facility | JPM Credit Facility, Portion Maturing February 11, 2026 | Line of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity				$ 37,500
Letter of Credit | JPM Credit Facility | Line of Credit
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity				$ 23,750