INFORMATION ABOUT RECEIVABLES, REVENUES, AND SERVICES - Investment Advisory Services (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Revenue and Services [Line Items]
Net revenues	$ 1,929.3	$ 1,415.4	$ 3,756.1	$ 2,878.0
Average assets under management	1,585,300.0	1,140,200.0	1,547,100.0	1,151,400.0
Period end assets under management	1,623,100.0		1,623,100.0		$ 1,470,500.0
U.S. mutual funds
Revenue and Services [Line Items]
Net revenues	1,209.3	918.2	2,367.3	1,899.5
Average assets under management	848,200.0	628,600.0	831,300.0	641,500.0
Period end assets under management	865,800.0		865,800.0		794,600.0
U.S. mutual funds \| Equity
Revenue and Services [Line Items]
Average assets under management	532,200.0	374,500.0	520,900.0	381,400.0
Period end assets under management	550,300.0		550,300.0		498,600.0
U.S. mutual funds \| Fixed income, including money market
Revenue and Services [Line Items]
Average assets under management	85,900.0	74,200.0	84,100.0	75,000.0
Period end assets under management	86,200.0		86,200.0		79,400.0
U.S. mutual funds \| Multi-asset
Revenue and Services [Line Items]
Average assets under management	230,100.0	179,900.0	226,300.0	185,100.0
Period end assets under management	229,300.0		229,300.0		216,600.0
Subadvised funds, separate accounts, collective investment trusts, and other investment products
Revenue and Services [Line Items]
Net revenues	689.7	470.7	1,327.3	922.3
Average assets under management	737,100.0	511,600.0	715,800.0	509,900.0
Period end assets under management	757,300.0		757,300.0		675,900.0
Subadvised funds, separate accounts, collective investment trusts, and other investment products \| Equity
Revenue and Services [Line Items]
Average assets under management	427,700.0	293,200.0	418,100.0	285,300.0
Period end assets under management	434,800.0		434,800.0		397,200.0
Subadvised funds, separate accounts, collective investment trusts, and other investment products \| Fixed income, including money market
Revenue and Services [Line Items]
Average assets under management	91,300.0	77,100.0	91,000.0	79,200.0
Period end assets under management	92,500.0		92,500.0		89,300.0
Subadvised funds, separate accounts, collective investment trusts, and other investment products \| Multi-asset
Revenue and Services [Line Items]
Average assets under management	218,100.0	141,300.0	206,700.0	145,400.0
Period end assets under management	230,000.0		230,000.0		$ 189,400.0
Other clients
Revenue and Services [Line Items]
Net revenues	30.3	26.5	61.5	56.2
Investment advisory fees
Revenue and Services [Line Items]
Net revenues	1,787.2	1,293.8	3,475.0	2,621.6
Investment advisory fees \| U.S. mutual funds
Revenue and Services [Line Items]
Net revenues	1,097.5	823.1	2,147.7	1,699.3
Investment advisory fees \| U.S. mutual funds \| Equity
Revenue and Services [Line Items]
Net revenues	768.0	545.9	1,497.8	1,114.3
Investment advisory fees \| U.S. mutual funds \| Fixed income, including money market
Revenue and Services [Line Items]
Net revenues	59.9	66.8	118.9	138.5
Investment advisory fees \| U.S. mutual funds \| Multi-asset
Revenue and Services [Line Items]
Net revenues	269.6	210.4	531.0	446.5
Investment advisory fees \| Subadvised funds, separate accounts, collective investment trusts, and other investment products
Revenue and Services [Line Items]
Net revenues	689.7	470.7	1,327.3	922.3
Investment advisory fees \| Subadvised funds, separate accounts, collective investment trusts, and other investment products \| Equity
Revenue and Services [Line Items]
Net revenues	446.0	305.6	870.2	585.1
Investment advisory fees \| Subadvised funds, separate accounts, collective investment trusts, and other investment products \| Fixed income, including money market
Revenue and Services [Line Items]
Net revenues	40.8	33.9	78.3	71.9
Investment advisory fees \| Subadvised funds, separate accounts, collective investment trusts, and other investment products \| Multi-asset
Revenue and Services [Line Items]
Net revenues	202.9	131.2	378.8	265.3
Investment advisory fees \| Other clients
Revenue and Services [Line Items]
Net revenues	0.0	0.0	0.0	0.0
Administrative fees
Revenue and Services [Line Items]
Net revenues	112.1	95.9	222.0	202.8
Administrative fees \| U.S. mutual funds
Revenue and Services [Line Items]
Net revenues	81.8	69.4	160.5	146.6
Administrative fees \| Subadvised funds, separate accounts, collective investment trusts, and other investment products
Revenue and Services [Line Items]
Net revenues	0.0	0.0	0.0	0.0
Administrative fees \| Other clients
Revenue and Services [Line Items]
Net revenues	30.3	26.5	61.5	56.2
Distribution and servicing fees
Revenue and Services [Line Items]
Net revenues	30.0	25.7	59.1	53.6
Distribution and servicing fees \| U.S. mutual funds
Revenue and Services [Line Items]
Net revenues	30.0	25.7	59.1	53.6
Distribution and servicing fees \| Subadvised funds, separate accounts, collective investment trusts, and other investment products
Revenue and Services [Line Items]
Net revenues	0.0	0.0	0.0	0.0
Distribution and servicing fees \| Other clients
Revenue and Services [Line Items]
Net revenues	$ 0.0	$ 0.0	$ 0.0	$ 0.0