INVESTMENTS - Carrying Value of Investments (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Schedule of Equity Method Investments [Line Items]
Investment, Type [Extensible Enumeration]	Certificates of Deposit [Member]	Certificates of Deposit [Member]
Held to maturity	$ 50.4	$ 44.7
Total	3,417.4	3,000.5
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	572.4	258.8
Equity method investments	20.2	60.8
Seed capital
Schedule of Equity Method Investments [Line Items]
Investments held at fair value	345.7	262.8
Equity method investments	37.5	128.8
Deferred compensation liabilities economic hedges
Schedule of Equity Method Investments [Line Items]
Investments held at fair value	1,060.2	992.8
Equity method investments	50.6	88.4
Investment partnerships and other investments
Schedule of Equity Method Investments [Line Items]
Investments held at fair value	180.3	62.6
Investments in affiliated collateralized loan obligations
Schedule of Equity Method Investments [Line Items]
Investments held at fair value	3.8	6.3
Held to maturity	34.4	61.1
Investment in UTI Asset Management Company Limited (India)
Schedule of Equity Method Investments [Line Items]
Equity method investments	165.7	173.5
Investments in affiliated private investment funds - carried interest
Schedule of Equity Method Investments [Line Items]
Equity method investments	413.1	426.9
Investments in affiliated private investment funds - seed/co-investment
Schedule of Equity Method Investments [Line Items]
Equity method investments	292.7	269.9
Investment partnerships and other investments
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 189.4	$ 162.1