Consolidated Investment Products - Schedule of Balances of Assets and Liabilities of CIPs Measured at Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2025	Sep. 30, 2024
Assets [Abstract]
Investments, at fair value	$ 636.4	$ 838.0
CIPs [Member]
Assets [Abstract]
Investments, at fair value	11,859.6	11,034.9
Fair Value, Recurring [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	596.7	838.0
Fair Value, Recurring [Member] \| Level 1 [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	294.1	552.9
Fair Value, Recurring [Member] \| Level 2 [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	192.3	168.5
Fair Value, Recurring [Member] \| Level 3 [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	10.6	7.8
Fair Value, Recurring [Member] \| NAV as a Practical Expedient [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	99.7	108.8
Fair Value, Recurring [Member] \| CIPs [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	12,519.8	11,948.8
Fair Value, Recurring [Member] \| CIPs [Member] \| Level 1 [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	1,049.5	994.0
Fair Value, Recurring [Member] \| CIPs [Member] \| Level 2 [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	10,761.4	10,217.1
Fair Value, Recurring [Member] \| CIPs [Member] \| Level 3 [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	553.5	550.6
Fair Value, Recurring [Member] \| CIPs [Member] \| NAV as a Practical Expedient [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	155.4	187.1
Equity and debt securities [Member] \| Fair Value, Recurring [Member] \| CIPs [Member]
Assets [Abstract]
Investments, at fair value	2,543.3	1,856.3
Equity and debt securities [Member] \| Fair Value, Recurring [Member] \| CIPs [Member] \| Level 1 [Member]
Assets [Abstract]
Investments, at fair value	511.6	229.7
Equity and debt securities [Member] \| Fair Value, Recurring [Member] \| CIPs [Member] \| Level 2 [Member]
Assets [Abstract]
Investments, at fair value	1,331.3	889.4
Equity and debt securities [Member] \| Fair Value, Recurring [Member] \| CIPs [Member] \| Level 3 [Member]
Assets [Abstract]
Investments, at fair value	545.0	550.1
Equity and debt securities [Member] \| Fair Value, Recurring [Member] \| CIPs [Member] \| NAV as a Practical Expedient [Member]
Assets [Abstract]
Investments, at fair value	155.4	187.1
Loans [Member] \| Fair Value, Recurring [Member] \| CIPs [Member]
Assets [Abstract]
Investments, at fair value	9,316.3	9,178.6
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	9,307.8	9,178.1
Loans [Member] \| Fair Value, Recurring [Member] \| CIPs [Member] \| Level 3 [Member]
Assets [Abstract]
Investments, at fair value	8.5	0.5
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	9,490.7	9,352.1
Collateralized loan obligations [Member] \| Fair Value, Recurring [Member]
Assets [Abstract]
Cash and cash equivalents of CLOs	537.9	764.3
Receivables of CLOs	122.3	149.6
Collateralized loan obligations [Member] \| Fair Value, Recurring [Member] \| Level 1 [Member]
Assets [Abstract]
Cash and cash equivalents of CLOs	537.9	764.3
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	122.3	149.6
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 asset recognized for present right to economic benefit.

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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.

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Fair value portion of receivables, including, but not limited to, trade account receivables, note receivables, and loan receivables.

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