INFORMATION ABOUT RECEIVABLES, REVENUES, AND SERVICES - Narrative (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2025	Jun. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Concentration Risk [Line Items]
Net revenues	$ 1,893.5		$ 1,785.6	$ 5,380.7	$ 5,269.1
Accounts receivable	899.4			899.4		$ 877.4
Sponsored investment portfolios
Concentration Risk [Line Items]
Net revenues	1,594.2		$ 1,486.5	4,500.7	$ 4,359.5
Accounts receivable	$ 628.2			$ 628.2		$ 602.0
Investment advisory fees | Assets under management | Geographic concentration | Outside U.S.
Concentration Risk [Line Items]
Concentration risk	8.70%	8.70%				8.80%