INFORMATION ABOUT RECEIVABLES, REVENUES, AND SERVICES - Investment Advisory Services (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Revenue and Services [Line Items]
Net revenues	$ 1,954.1	$ 1,595.8	$ 5,710.2	$ 4,473.8
Average assets under management	1,648,700.0	1,292,900.0	1,581,300.0	1,198,900.0
Period end assets under management	1,612,300.0		1,612,300.0		$ 1,470,500.0
U.S. mutual funds
Revenue and Services [Line Items]
Net revenues	1,239.0	1,040.4	3,606.3	2,939.9
Average assets under management	881,000.0	708,800.0	848,100.0	664,000.0
Period end assets under management	861,000.0		861,000.0		794,600.0
U.S. mutual funds \| Equity
Revenue and Services [Line Items]
Average assets under management	559,800.0	433,400.0	534,000.0	398,900.0
Period end assets under management	546,100.0		546,100.0		498,600.0
U.S. mutual funds \| Fixed income, including money market
Revenue and Services [Line Items]
Average assets under management	88,400.0	78,100.0	85,600.0	76,000.0
Period end assets under management	87,700.0		87,700.0		79,400.0
U.S. mutual funds \| Multi-asset
Revenue and Services [Line Items]
Average assets under management	232,800.0	197,300.0	228,500.0	189,100.0
Period end assets under management	227,200.0		227,200.0		216,600.0
Subadvised funds, separate accounts, collective investment trusts, and other investment products
Revenue and Services [Line Items]
Net revenues	687.9	529.8	2,015.2	1,452.1
Average assets under management	767,700.0	584,100.0	733,200.0	534,900.0
Period end assets under management	751,300.0		751,300.0		675,900.0
Subadvised funds, separate accounts, collective investment trusts, and other investment products \| Equity
Revenue and Services [Line Items]
Average assets under management	445,700.0	338,800.0	427,300.0	303,300.0
Period end assets under management	428,600.0		428,600.0		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	94,300.0	82,400.0	92,100.0	80,300.0
Period end assets under management	93,000.0		93,000.0		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	227,700.0	162,900.0	213,800.0	151,300.0
Period end assets under management	229,700.0		229,700.0		$ 189,400.0
Other clients
Revenue and Services [Line Items]
Net revenues	27.2	25.6	88.7	81.8
Investment advisory fees
Revenue and Services [Line Items]
Net revenues	1,813.4	1,469.3	5,288.4	4,090.9
Investment advisory fees \| U.S. mutual funds
Revenue and Services [Line Items]
Net revenues	1,125.5	939.5	3,273.2	2,638.8
Investment advisory fees \| U.S. mutual funds \| Equity
Revenue and Services [Line Items]
Net revenues	813.0	636.3	2,310.8	1,750.6
Investment advisory fees \| U.S. mutual funds \| Fixed income, including money market
Revenue and Services [Line Items]
Net revenues	64.8	66.6	183.7	205.1
Investment advisory fees \| U.S. mutual funds \| Multi-asset
Revenue and Services [Line Items]
Net revenues	247.7	236.6	778.7	683.1
Investment advisory fees \| Subadvised funds, separate accounts, collective investment trusts, and other investment products
Revenue and Services [Line Items]
Net revenues	687.9	529.8	2,015.2	1,452.1
Investment advisory fees \| Subadvised funds, separate accounts, collective investment trusts, and other investment products \| Equity
Revenue and Services [Line Items]
Net revenues	458.8	339.6	1,329.0	924.7
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	41.3	38.4	119.6	110.3
Investment advisory fees \| Subadvised funds, separate accounts, collective investment trusts, and other investment products \| Multi-asset
Revenue and Services [Line Items]
Net revenues	187.8	151.8	566.6	417.1
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	109.8	98.0	331.8	300.8
Administrative fees \| U.S. mutual funds
Revenue and Services [Line Items]
Net revenues	82.6	72.4	243.1	219.0
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	27.2	25.6	88.7	81.8
Distribution and servicing fees
Revenue and Services [Line Items]
Net revenues	30.9	28.5	90.0	82.1
Distribution and servicing fees \| U.S. mutual funds
Revenue and Services [Line Items]
Net revenues	30.9	28.5	90.0	82.1
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