INFORMATION ABOUT RECEIVABLES, REVENUES, AND SERVICES. (Investment Advisory Services) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Revenue and Services [Line Items]
Net revenues	$ 1,826.8	$ 1,462.6
Average assets under management	1,508,800.0	1,162,700.0
Period end assets under management	1,518,000.0		$ 1,470,500.0
U.S. mutual funds
Revenue and Services [Line Items]
Net revenues	1,158.0	981.3
Average assets under management	814,500.0	654,300.0
Period end assets under management	816,400.0		794,600.0
U.S. mutual funds \| Equity
Revenue and Services [Line Items]
Average assets under management	509,600.0	388,300.0
Period end assets under management	511,700.0		498,600.0
U.S. mutual funds \| Fixed income, including money market
Revenue and Services [Line Items]
Average assets under management	82,300.0	75,700.0
Period end assets under management	82,600.0		79,400.0
U.S. mutual funds \| Multi-asset
Revenue and Services [Line Items]
Average assets under management	222,600.0	190,300.0
Period end assets under management	222,100.0		216,600.0
Subadvised funds, separate accounts, collective investment trusts, and other investment products
Revenue and Services [Line Items]
Net revenues	637.6	451.6
Average assets under management	694,300.0	508,400.0
Period end assets under management	701,600.0		675,900.0
Subadvised funds, separate accounts, collective investment trusts, and other investment products \| Equity
Revenue and Services [Line Items]
Average assets under management	418,900.0	277,500.0
Period end assets under management	405,500.0		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	80,200.0	81,300.0
Period end assets under management	90,200.0		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	195,200.0	149,600.0
Period end assets under management	205,900.0		$ 189,400.0
Other clients
Revenue and Services [Line Items]
Net revenues	31.2	29.7
Investment advisory fees
Revenue and Services [Line Items]
Net revenues	1,687.8	1,327.8
Investment advisory fees \| U.S. mutual funds
Revenue and Services [Line Items]
Net revenues	1,050.2	876.2
Investment advisory fees \| U.S. mutual funds \| Equity
Revenue and Services [Line Items]
Net revenues	729.8	568.4
Investment advisory fees \| U.S. mutual funds \| Fixed income, including money market
Revenue and Services [Line Items]
Net revenues	59.0	71.7
Investment advisory fees \| U.S. mutual funds \| Multi-asset
Revenue and Services [Line Items]
Net revenues	261.4	236.1
Investment advisory fees \| Subadvised funds, separate accounts, collective investment trusts, and other investment products
Revenue and Services [Line Items]
Net revenues	637.6	451.6
Investment advisory fees \| Subadvised funds, separate accounts, collective investment trusts, and other investment products \| Equity
Revenue and Services [Line Items]
Net revenues	429.8	279.5
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	31.7	38.0
Investment advisory fees \| Subadvised funds, separate accounts, collective investment trusts, and other investment products \| Multi-asset
Revenue and Services [Line Items]
Net revenues	176.1	134.1
Investment advisory fees \| Other clients
Revenue and Services [Line Items]
Net revenues	0.0	0.0
Administrative fees
Revenue and Services [Line Items]
Net revenues	109.9	106.9
Administrative fees \| U.S. mutual funds
Revenue and Services [Line Items]
Net revenues	78.7	77.2
Administrative fees \| Subadvised funds, separate accounts, collective investment trusts, and other investment products
Revenue and Services [Line Items]
Net revenues	0.0	0.0
Administrative fees \| Other clients
Revenue and Services [Line Items]
Net revenues	31.2	29.7
Distribution and servicing fees
Revenue and Services [Line Items]
Net revenues	29.1	27.9
Distribution and servicing fees \| U.S. mutual funds
Revenue and Services [Line Items]
Net revenues	29.1	27.9
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
Distribution and servicing fees \| Other clients
Revenue and Services [Line Items]
Net revenues	$ 0.0	$ 0.0