INFORMATION ABOUT RECEIVABLES, REVENUES, AND SERVICES - Investment Advisory Services (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Revenue and Services [Line Items]
Net revenues	$ 1,893.5	$ 1,785.6	$ 5,380.7	$ 5,269.1
Investment advisory fees
Revenue and Services [Line Items]
Net revenues	1,698.7	1,627.3	4,864.7	4,732.5
Investment advisory fees | Assets under management
Revenue and Services [Line Items]
Net revenues	1,723,000.0	1,589,500.0	1,644,400.0	1,536,200.0
Performance-based advisory fees
Revenue and Services [Line Items]
Net revenues	6.4	5.6	23.2	40.0
Capital allocation-based income (loss)
Revenue and Services [Line Items]
Net revenues	42.0	4.6	40.4	51.8
Administrative, distribution, services, and other fees
Revenue and Services [Line Items]
Net revenues	146.4	148.1	452.4	444.8
Equity | Investment advisory fees | Assets under management
Revenue and Services [Line Items]
Net revenues	865,800.0	813,100.0	825,700.0	791,400.0
Fixed income, including money market | Investment advisory fees | Assets under management
Revenue and Services [Line Items]
Net revenues	203,800.0	183,300.0	197,800.0	175,900.0
Multi-asset | Investment advisory fees | Assets under management
Revenue and Services [Line Items]
Net revenues	597,700.0	542,300.0	566,800.0	519,900.0
Alternatives | Investment advisory fees | Assets under management
Revenue and Services [Line Items]
Net revenues	55,700.0	50,800.0	54,100.0	49,000.0
Investment advisory fees | Investment advisory fees
Revenue and Services [Line Items]
Net revenues	1,698.7	1,627.3	4,864.7	4,732.5
Investment advisory fees | Equity | Investment advisory fees
Revenue and Services [Line Items]
Net revenues	1,011.8	978.5	2,894.6	2,858.5
Investment advisory fees | Fixed income, including money market | Investment advisory fees
Revenue and Services [Line Items]
Net revenues	110.1	104.1	319.2	304.5
Investment advisory fees | Multi-asset | Investment advisory fees
Revenue and Services [Line Items]
Net revenues	492.1	465.8	1,402.7	1,340.3
Investment advisory fees | Alternatives | Investment advisory fees
Revenue and Services [Line Items]
Net revenues	$ 84.7	$ 78.9	$ 248.2	$ 229.2