Fair Value Measurements - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Net change in unrealized appreciation (depreciation) on investment transactions	$ (3,425)	$ (11,391)	$ (152,087)	$ 22,401
Level 3
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Net change in unrealized appreciation (depreciation) on investment transactions	$ (52,892)	$ 41,204	$ (205,947)	$ 10,964

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/1943274/2147477346/946-830-45-36

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/1943274/2147479134/946-220-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/1943274/2147479105/946-220-45-6

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