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Fair Value Measurements - Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Millions
Mar. 31, 2025
Sep. 30, 2024
Liability [Abstract]    
Equity Securities without Readily Determinable Fair Value, Amount [1] $ 39.7  
Sponsored funds and separate accounts [Member]    
Assets [Abstract]    
Investments, at fair value 278.0 $ 509.1
Investments related to long-term incentive plans [Member]    
Assets [Abstract]    
Investments, at fair value 258.2 271.6
Other equity and debt investments [Member]    
Assets [Abstract]    
Investments, at fair value 100.2 57.3
Fair Value, Recurring [Member]    
Assets [Abstract]    
Total Assets Measured at Fair Value 596.7 838.0
Liability [Abstract]    
Securities sold short 116.6 178.1
Contingent consideration liabilities 27.5 28.2
Total Liabilities Measured at Fair Value 144.1 206.3
Fair Value, Recurring [Member] | Level 1 [Member]    
Assets [Abstract]    
Total Assets Measured at Fair Value 294.1 552.9
Liability [Abstract]    
Securities sold short 116.6 178.1
Contingent consideration liabilities 0.0 0.0
Total Liabilities Measured at Fair Value 116.6 178.1
Fair Value, Recurring [Member] | Level 2 [Member]    
Assets [Abstract]    
Total Assets Measured at Fair Value 192.3 168.5
Liability [Abstract]    
Securities sold short 0.0 0.0
Contingent consideration liabilities 0.0 0.0
Total Liabilities Measured at Fair Value 0.0 0.0
Fair Value, Recurring [Member] | Level 3 [Member]    
Assets [Abstract]    
Total Assets Measured at Fair Value 10.6 7.8
Liability [Abstract]    
Securities sold short 0.0 0.0
Contingent consideration liabilities 27.5 28.2
Total Liabilities Measured at Fair Value 27.5 28.2
Fair Value, Recurring [Member] | NAV as a Practical Expedient [Member]    
Assets [Abstract]    
Total Assets Measured at Fair Value 99.7 108.8
Liability [Abstract]    
Securities sold short 0.0 0.0
Contingent consideration liabilities 0.0 0.0
Total Liabilities Measured at Fair Value 0.0 0.0
Fair Value, Recurring [Member] | Sponsored funds and separate accounts [Member]    
Assets [Abstract]    
Investments, at fair value 278.0 509.1
Fair Value, Recurring [Member] | Sponsored funds and separate accounts [Member] | Level 1 [Member]    
Assets [Abstract]    
Investments, at fair value 54.7 306.3
Fair Value, Recurring [Member] | Sponsored funds and separate accounts [Member] | Level 2 [Member]    
Assets [Abstract]    
Investments, at fair value 178.0 157.4
Fair Value, Recurring [Member] | Sponsored funds and separate accounts [Member] | Level 3 [Member]    
Assets [Abstract]    
Investments, at fair value 9.0 5.2
Fair Value, Recurring [Member] | Sponsored funds and separate accounts [Member] | NAV as a Practical Expedient [Member]    
Assets [Abstract]    
Investments, at fair value 36.3 40.2
Fair Value, Recurring [Member] | Investments related to long-term incentive plans [Member]    
Assets [Abstract]    
Investments, at fair value 258.2 271.6
Fair Value, Recurring [Member] | Investments related to long-term incentive plans [Member] | Level 1 [Member]    
Assets [Abstract]    
Investments, at fair value 226.2 242.5
Fair Value, Recurring [Member] | Investments related to long-term incentive plans [Member] | Level 2 [Member]    
Assets [Abstract]    
Investments, at fair value 2.8 0.0
Fair Value, Recurring [Member] | Investments related to long-term incentive plans [Member] | Level 3 [Member]    
Assets [Abstract]    
Investments, at fair value 0.0 0.0
Fair Value, Recurring [Member] | Investments related to long-term incentive plans [Member] | NAV as a Practical Expedient [Member]    
Assets [Abstract]    
Investments, at fair value 29.2 29.1
Fair Value, Recurring [Member] | Other equity and debt investments [Member]    
Assets [Abstract]    
Investments, at fair value 60.5 57.3
Fair Value, Recurring [Member] | Other equity and debt investments [Member] | Level 1 [Member]    
Assets [Abstract]    
Investments, at fair value 13.2 4.1
Fair Value, Recurring [Member] | Other equity and debt investments [Member] | Level 2 [Member]    
Assets [Abstract]    
Investments, at fair value 11.5 11.1
Fair Value, Recurring [Member] | Other equity and debt investments [Member] | Level 3 [Member]    
Assets [Abstract]    
Investments, at fair value 1.6 2.6
Fair Value, Recurring [Member] | Other equity and debt investments [Member] | NAV as a Practical Expedient [Member]    
Assets [Abstract]    
Investments, at fair value $ 34.2 $ 39.5
[1]
As of March 31, 2025, there were $39.7 million of other investments which were adjusted to fair value on a nonrecurring basis and excluded from the table above.