Consolidated Investment Products - Schedule of Unpaid Principal Balance and Fair Value of Investments (Details) - USD ($) $ in Millions	Mar. 31, 2024	Sep. 30, 2023
Schedule Of Consolidated Investment Products [Line Items]
Unpaid principal balance	$ 2,377.7	$ 2,222.0
Total investments, at fair value	899.7	872.8
Collateralized loan obligations [Member]
Schedule Of Consolidated Investment Products [Line Items]
Unpaid principal balance	9,365.9	8,317.5
Difference between unpaid principal balance and fair value	22.4	(120.7)
Total investments, at fair value	$ 9,388.3	$ 8,196.8

- Definition

[Line Items] for Tabular disclosure of qualitative and quantitative information related to investment products holds, whether or not such investment products (CIP) is included in the reporting entity's consolidated financial statements. Includes, but is not limited to, description of the significant judgments and assumptions made in determining whether a CIP (as defined) held by the entity requires the CIP (as defined) to be consolidated and (or) disclose information about its involvement with the CIP, individually or in aggregate (as applicable); the nature of restrictions, if any, on the CIP's assets and on the settlement of its liabilities reported by an entity in its statement of financial position, including the carrying amounts of such assets and liabilities; the nature of, and changes in, the risks associated with involvement in the CIP; how involvement with the CIP affects the entity's financial position, financial performance, and cash flows; the lack of recourse if creditors (or beneficial interest holders) of the CIP have no recourse to the general credit of the primary beneficiary (if applicable); the terms of arrangements, giving consideration to both explicit arrangements and implicit variable interests, if any, that could require the entity to provide financial support to the SIP, including events or circumstances that could expose the entity to a loss; the methodology used by the entity for determining whether or not it is the primary beneficiary of the consolidated investment products; the significant factors considered and judgments made in determining that the power to direct the activities of a CIP that most significantly impact the CIP's economic performance are shared (as defined); the carrying amounts and classification of assets and liabilities of the CIP included in the statement of financial position.

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