Fair Value Measurements - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Net change in unrealized appreciation (depreciation) on investment transactions	$ 41,050	$ (40,555)	$ (5,558)	$ (13,162)
Level 3
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Net change in unrealized appreciation (depreciation) on investment transactions	$ 9,785	$ (63,019)	$ (293)	$ (35,920)

Amount, before tax, of unrealized gain (loss) on investment, derivative, and foreign currency transaction on investment and derivative from change in market price, classified as operating. Investment includes, but is not limited to, debt and equity securities, security sold short, real estate, and commodity.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 830
-Name Accounting Standards Codification
-Section 45
-Paragraph 36
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480228/946-830-45-36

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07(7)(c))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483575/946-220-S99-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section 45
-Paragraph 6
-Subparagraph (b)(1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483581/946-220-45-6

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration