Consolidated Investment Products - Schedule of Balances of Assets and Liabilities of CIPs Measured at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2025	Sep. 30, 2024
Assets [Abstract]
Investments, at fair value	$ 1,179.5	$ 838.0
CIPs [Member]
Assets [Abstract]
Investments, at fair value	12,278.8	11,034.9
Fair Value, Recurring [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	1,150.5	838.0
Fair Value, Recurring [Member] \| Level 1 [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	729.7	552.9
Fair Value, Recurring [Member] \| Level 2 [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	317.9	168.5
Fair Value, Recurring [Member] \| Level 3 [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	4.0	7.8
Fair Value, Recurring [Member] \| NAV as a Practical Expedient [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	98.9	108.8
Fair Value, Recurring [Member] \| CIPs [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	12,863.9	11,948.8
Fair Value, Recurring [Member] \| CIPs [Member] \| Level 1 [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	865.3	994.0
Fair Value, Recurring [Member] \| CIPs [Member] \| Level 2 [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	11,198.6	10,217.1
Fair Value, Recurring [Member] \| CIPs [Member] \| Level 3 [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	610.9	550.6
Fair Value, Recurring [Member] \| CIPs [Member] \| NAV as a Practical Expedient [Member]
Assets [Abstract]
Total Assets Measured at Fair Value	189.1	187.1
Equity and debt securities [Member] \| Fair Value, Recurring [Member] \| CIPs [Member]
Assets [Abstract]
Investments, at fair value	1,914.8	1,856.3
Equity and debt securities [Member] \| Fair Value, Recurring [Member] \| CIPs [Member] \| Level 1 [Member]
Assets [Abstract]
Investments, at fair value	393.2	229.7
Equity and debt securities [Member] \| Fair Value, Recurring [Member] \| CIPs [Member] \| Level 2 [Member]
Assets [Abstract]
Investments, at fair value	731.5	889.4
Equity and debt securities [Member] \| Fair Value, Recurring [Member] \| CIPs [Member] \| Level 3 [Member]
Assets [Abstract]
Investments, at fair value	601.0	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	189.1	187.1
Loans [Member] \| Fair Value, Recurring [Member] \| CIPs [Member]
Assets [Abstract]
Investments, at fair value	10,364.0	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	10,354.1	9,178.1
Loans [Member] \| Fair Value, Recurring [Member] \| CIPs [Member] \| Level 3 [Member]
Assets [Abstract]
Investments, at fair value	9.9	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	10,597.7	9,352.1
Collateralized Loan Obligations [Member] \| Fair Value, Recurring [Member]
Assets [Abstract]
Cash and cash equivalents of CLOs	472.1	764.3
Receivables of CLOs	113.0	149.6
Collateralized Loan Obligations [Member] \| Fair Value, Recurring [Member] \| Level 1 [Member]
Assets [Abstract]
Cash and cash equivalents of CLOs	472.1	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	113.0	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