INFORMATION ABOUT RECEIVABLES, REVENUES, AND SERVICES. (Investment Advisory Services) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Revenue and Services [Line Items]
Net revenues	$ 1,595.8	$ 1,426.7	$ 4,473.8	$ 4,149.2
Average assets under management	1,292,900.0	1,130,200.0	1,198,900.0	1,091,500.0
Period end assets under management	1,310,400.0		1,310,400.0		$ 1,206,800.0
U.S. mutual funds
Revenue and Services [Line Items]
Net revenues	1,040.4	980.1	2,939.9	2,863.3
Average assets under management	708,800.0	650,500.0	664,000.0	633,200.0
Period end assets under management	716,200.0		716,200.0		682,700.0
U.S. mutual funds | Equity
Revenue and Services [Line Items]
Average assets under management	433,400.0	384,400.0	398,900.0	371,500.0
Period end assets under management	442,300.0		442,300.0		407,100.0
U.S. mutual funds | Fixed income, including money market
Revenue and Services [Line Items]
Average assets under management	78,100.0	73,300.0	76,000.0	71,300.0
Period end assets under management	77,600.0		77,600.0		73,700.0
U.S. mutual funds | Multi-asset
Revenue and Services [Line Items]
Average assets under management	197,300.0	192,800.0	189,100.0	190,400.0
Period end assets under management	196,300.0		196,300.0		201,900.0
Subadvised funds, separate accounts, collective investment trusts, and other investment products
Revenue and Services [Line Items]
Net revenues	529.8	422.4	1,452.1	1,210.2
Average assets under management	584,100.0	479,700.0	534,900.0	458,300.0
Period end assets under management	594,200.0		594,200.0		524,100.0
Subadvised funds, separate accounts, collective investment trusts, and other investment products | Equity
Revenue and Services [Line Items]
Average assets under management	338,800.0	262,400.0	303,300.0	251,100.0
Period end assets under management	343,500.0		343,500.0		291,700.0
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	82,400.0	80,900.0	80,300.0	78,900.0
Period end assets under management	83,500.0		83,500.0		74,200.0
Subadvised funds, separate accounts, collective investment trusts, and other investment products | Multi-asset
Revenue and Services [Line Items]
Average assets under management	162,900.0	136,400.0	151,300.0	128,300.0
Period end assets under management	167,200.0		167,200.0		$ 158,200.0
Other clients
Revenue and Services [Line Items]
Net revenues	25.6	24.2	81.8	75.7
Investment advisory fees
Revenue and Services [Line Items]
Net revenues	1,469.3	1,303.4	4,090.9	3,767.8
Investment advisory fees | U.S. mutual funds
Revenue and Services [Line Items]
Net revenues	939.5	881.0	2,638.8	2,557.6
Investment advisory fees | U.S. mutual funds | Equity
Revenue and Services [Line Items]
Net revenues	636.3	570.2	1,750.6	1,641.4
Investment advisory fees | U.S. mutual funds | Fixed income, including money market
Revenue and Services [Line Items]
Net revenues	66.6	70.7	205.1	208.6
Investment advisory fees | U.S. mutual funds | Multi-asset
Revenue and Services [Line Items]
Net revenues	236.6	240.1	683.1	707.6
Investment advisory fees | Subadvised funds, separate accounts, collective investment trusts, and other investment products
Revenue and Services [Line Items]
Net revenues	529.8	422.4	1,452.1	1,210.2
Investment advisory fees | Subadvised funds, separate accounts, collective investment trusts, and other investment products | Equity
Revenue and Services [Line Items]
Net revenues	339.6	259.5	924.7	751.0
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	38.4	39.7	110.3	116.0
Investment advisory fees | Subadvised funds, separate accounts, collective investment trusts, and other investment products | Multi-asset
Revenue and Services [Line Items]
Net revenues	151.8	123.2	417.1	343.2
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	98.0	93.2	300.8	291.0
Administrative fees | U.S. mutual funds
Revenue and Services [Line Items]
Net revenues	72.4	69.0	219.0	215.3
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	25.6	24.2	81.8	75.7
Distribution and servicing fees
Revenue and Services [Line Items]
Net revenues	28.5	30.1	82.1	90.4
Distribution and servicing fees | U.S. mutual funds
Revenue and Services [Line Items]
Net revenues	28.5	30.1	82.1	90.4
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