Commitments and Contingencies - Schedule of Unfunded Commitments by Investment (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Sep. 30, 2023
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	$ 300,608	$ 224,611
Investment, Identifier [Axis]: 107 Fair Street LLC
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	3,434	3,434
Investment, Identifier [Axis]: 107-109 Beech OAK22 LLC
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	16,983	16,983
Investment, Identifier [Axis]: 112-126 Van Houten Real22 LLC
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	1,728	1,892
Investment, Identifier [Axis]: ACP Falcon Buyer Inc
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	5,333	5,333
Investment, Identifier [Axis]: ADC Therapeutics SA
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	4,770	4,770
Investment, Identifier [Axis]: ASP-R-PAC Acquisition Co LLC
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	588	588
Investment, Identifier [Axis]: AmSpec Parent LLC
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	19,666	0
Investment, Identifier [Axis]: Ardonagh Midco 3 PLC
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	3,520	3,520
Investment, Identifier [Axis]: Avalara, Inc.
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	1,903	1,903
Investment, Identifier [Axis]: Bamboo US Bidco LLC
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	8,832	9,100
Investment, Identifier [Axis]: BioXcel Therapeutics, Inc.
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	4,471	6,932
Investment, Identifier [Axis]: CVAUSA Management, LLC
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	13,657	17,469
Investment, Identifier [Axis]: Coupa Holdings, LLC
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	2,122	2,122
Investment, Identifier [Axis]: Crewline Buyer, Inc.
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	4,573	0
Investment, Identifier [Axis]: Delta Leasing SPV II LLC
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	0	11,560
Investment, Identifier [Axis]: Dukes Root Control Inc.
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	2,535	3,104
Investment, Identifier [Axis]: Entrata, Inc.
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	5,211	5,211
Investment, Identifier [Axis]: Enverus Holdings, Inc.
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	6,830	0
Investment, Identifier [Axis]: Establishment Labs Holdings Inc.
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	3,378	3,378
Investment, Identifier [Axis]: Evergreen IX Borrower 2023, LLC
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	4,006	4,006
Investment, Identifier [Axis]: Finastra USA, Inc.
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	3,336	3,577
Investment, Identifier [Axis]: Galileo Parent, Inc.
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	2,118	1,757
Investment, Identifier [Axis]: Grove Hotel Parcel Owner, LLC
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	5,305	5,305
Investment, Identifier [Axis]: HUB Pen Company, LLC
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	3,213	3,213
Investment, Identifier [Axis]: Harrow, Inc.
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	5,018	5,018
Investment, Identifier [Axis]: IW Buyer LLC
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	6,432	6,432
Investment, Identifier [Axis]: Impel Pharmaceuticals Inc.
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	0	894
Investment, Identifier [Axis]: Innocoll Pharmaceuticals Limited
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	0	2,656
Investment, Identifier [Axis]: Inventus Power, Inc.
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	4,967	4,967
Investment, Identifier [Axis]: Kings Buyer, LLC
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	5,549	5,471
Investment, Identifier [Axis]: LSL Holdco, LLC
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	1,015	1,015
Investment, Identifier [Axis]: MND Holdings III Corp
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	2,333	9,331
Investment, Identifier [Axis]: MRI Software LLC
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	25,000	0
Investment, Identifier [Axis]: NFM & J, L.P.
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	17,360	0
Investment, Identifier [Axis]: Next Holdco, LLC
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	16,443	0
Investment, Identifier [Axis]: North Star Acquisitionco, LLC
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	7,732	7,732
Investment, Identifier [Axis]: OneOncology, LLC
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	13,159	13,159
Investment, Identifier [Axis]: Oranje Holdco, Inc.
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	1,968	1,968
Investment, Identifier [Axis]: PPW Aero Buyer, Inc.
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	3,363	3,603
Investment, Identifier [Axis]: PetVet Care Centers, LLC
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	19,210	0
Investment, Identifier [Axis]: Pluralsight, LLC
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	366	611
Investment, Identifier [Axis]: Resistance Acquisition, Inc.
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	0	10,507
Investment, Identifier [Axis]: SCP Eye Care Services, LLC
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	1,003	1,730
Investment, Identifier [Axis]: SEI Holding I Corporation
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	7,163	2,633
Investment, Identifier [Axis]: Salus Workers' Compensation, LLC
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	1,898	1,898
Investment, Identifier [Axis]: Seres Therapeutics, Inc.
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	12,990	12,990
Investment, Identifier [Axis]: Supreme Fitness Group NY Holdings, LLC
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	0	561
Investment, Identifier [Axis]: Transit Buyer LLC
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	2,214	3,850
Investment, Identifier [Axis]: WP CPP Holdings, LLC
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	5,831	0
Investment, Identifier [Axis]: iCIMs, Inc.
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	4,428	4,774
Investment, Identifier [Axis]: scPharmaceuticals Inc.
Financial Support for Nonconsolidated Legal Entity [Line Items]
Unfunded commitments	$ 7,654	$ 7,654