Consolidated Investment Products - Schedule of Balances of Assets and Liabilities of CIPs Measured at Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2024	Sep. 30, 2023
Assets [Abstract]
Investments, at fair value	$ 899.7	$ 872.8
CIPs [Member]
Assets [Abstract]
Investments, at fair value	11,149.1	9,637.2
Fair Value, Recurring [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	899.7	872.8
Fair Value, Recurring [Member] \| Level 1 [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	554.7	528.1
Fair Value, Recurring [Member] \| Level 2 [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	220.6	223.2
Fair Value, Recurring [Member] \| Level 3 [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	17.4	21.8
Fair Value, Recurring [Member] \| NAV as a Practical Expedient [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	107.0	99.7
Fair Value, Recurring [Member] \| CIPs [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	11,773.5	10,106.2
Fair Value, Recurring [Member] \| CIPs [Member] \| Level 1 [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	856.0	563.2
Fair Value, Recurring [Member] \| CIPs [Member] \| Level 2 [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	10,166.3	8,804.1
Fair Value, Recurring [Member] \| CIPs [Member] \| Level 3 [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	591.0	584.9
Fair Value, Recurring [Member] \| CIPs [Member] \| NAV as a Practical Expedient [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	160.2	154.0
Equity and debt securities [Member] \| Fair Value, Recurring [Member] \| CIPs [Member]
Assets [Abstract]
Investments, at fair value	1,924.7	1,592.4
Equity and debt securities [Member] \| Fair Value, Recurring [Member] \| CIPs [Member] \| Level 1 [Member]
Assets [Abstract]
Investments, at fair value	307.7	210.9
Equity and debt securities [Member] \| Fair Value, Recurring [Member] \| CIPs [Member] \| Level 2 [Member]
Assets [Abstract]
Investments, at fair value	865.8	642.6
Equity and debt securities [Member] \| Fair Value, Recurring [Member] \| CIPs [Member] \| Level 3 [Member]
Assets [Abstract]
Investments, at fair value	591.0	584.9
Equity and debt securities [Member] \| Fair Value, Recurring [Member] \| CIPs [Member] \| NAV as a Practical Expedient [Member]
Assets [Abstract]
Investments, at fair value	160.2	154.0
Loans [Member] \| Fair Value, Recurring [Member] \| CIPs [Member]
Assets [Abstract]
Investments, at fair value	9,224.4	8,044.8
Loans [Member] \| Fair Value, Recurring [Member] \| CIPs [Member] \| Level 1 [Member]
Assets [Abstract]
Investments, at fair value	0.0	0.0
Loans [Member] \| Fair Value, Recurring [Member] \| CIPs [Member] \| Level 2 [Member]
Assets [Abstract]
Investments, at fair value	9,224.4	8,044.8
Loans [Member] \| Fair Value, Recurring [Member] \| CIPs [Member] \| Level 3 [Member]
Assets [Abstract]
Investments, at fair value	0.0	0.0
Loans [Member] \| Fair Value, Recurring [Member] \| CIPs [Member] \| NAV as a Practical Expedient [Member]
Assets [Abstract]
Investments, at fair value	0.0	0.0
Collateralized loan obligations [Member]
Assets [Abstract]
Investments, at fair value	9,388.3	8,196.8
Collateralized loan obligations [Member] \| Fair Value, Recurring [Member]
Assets [Abstract]
Cash and cash equivalents of CLOs	548.3	352.3
Receivables of CLOs	76.1	116.7
Collateralized loan obligations [Member] \| Fair Value, Recurring [Member] \| Level 1 [Member]
Assets [Abstract]
Cash and cash equivalents of CLOs	548.3	352.3
Receivables of CLOs	0.0	0.0
Collateralized loan obligations [Member] \| Fair Value, Recurring [Member] \| Level 2 [Member]
Assets [Abstract]
Cash and cash equivalents of CLOs	0.0	0.0
Receivables of CLOs	76.1	116.7
Collateralized loan obligations [Member] \| Fair Value, Recurring [Member] \| Level 3 [Member]
Assets [Abstract]
Cash and cash equivalents of CLOs	0.0	0.0
Receivables of CLOs	0.0	0.0
Collateralized loan obligations [Member] \| Fair Value, Recurring [Member] \| NAV as a Practical Expedient [Member]
Assets [Abstract]
Cash and cash equivalents of CLOs	0.0	0.0
Receivables of CLOs	$ 0.0	$ 0.0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
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Fair value portion of receivables, including, but not limited to, trade account receivables, note receivables, and loan receivables.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 10
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482907/825-10-50-10

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-Name Accounting Standards Codification
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