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INVESTMENTS - Carrying Value of Investments (Details) - USD ($)
$ in Millions
Mar. 31, 2024
Dec. 31, 2023
Schedule of Equity Method Investments [Line Items]    
Investment, Type [Extensible Enumeration] Certificates of Deposit [Member] Certificates of Deposit [Member]
Held to maturity $ 25.4 $ 23.3
Total 2,588.7 2,554.7
U.S. Treasury note    
Schedule of Equity Method Investments [Line Items]    
Held to maturity 1.0 1.0
Discretionary investments    
Schedule of Equity Method Investments [Line Items]    
Investments held at fair value 259.9 246.4
Equity method investments 0.0 5.3
Seed capital    
Schedule of Equity Method Investments [Line Items]    
Investments held at fair value 237.7 247.8
Equity method investments 96.3 91.1
Supplemental savings plan liability economic hedges    
Schedule of Equity Method Investments [Line Items]    
Investments held at fair value 837.4 806.6
Equity method investments 24.5 21.0
Investment partnerships and other investments    
Schedule of Equity Method Investments [Line Items]    
Investments held at fair value 67.7 69.7
Investments in affiliated collateralized loan obligations    
Schedule of Equity Method Investments [Line Items]    
Investments held at fair value 7.9 8.4
Held to maturity $ 88.9 $ 94.1
23% Investment in UTI Asset Management Company Limited (India)    
Schedule of Equity Method Investments [Line Items]    
Equity method investment (ownership percentage) 23.00% 23.00%
Equity method investments $ 171.5 $ 164.5
Investments in affiliated private investment funds - carried interest    
Schedule of Equity Method Investments [Line Items]    
Equity method investments 514.3 519.9
Investments in affiliated private investment funds - seed/co-investment    
Schedule of Equity Method Investments [Line Items]    
Equity method investments 254.5 253.4
Investment Partnerships And Other Investments    
Schedule of Equity Method Investments [Line Items]    
Equity method investments $ 1.7 $ 2.2