INFORMATION ABOUT RECEIVABLES, REVENUES, AND SERVICES. (Investment Advisory Services) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Revenue and Services [Line Items]
Net revenues	$ 1,415.4	$ 1,395.2	$ 2,878.0	$ 2,722.5
Average assets under management	1,140,200.0	1,099,800.0	1,151,400.0	1,071,800.0
Period end assets under management	1,220,000.0		1,220,000.0		$ 1,206,800.0
U.S. mutual funds
Revenue and Services [Line Items]
Net revenues	918.2	960.5	1,899.5	1,883.2
Average assets under management	628,600.0	638,500.0	641,500.0	624,400.0
Period end assets under management	670,800.0		670,800.0		682,700.0
U.S. mutual funds | Equity
Revenue and Services [Line Items]
Average assets under management	374,500.0	375,800.0	381,400.0	365,100.0
Period end assets under management	407,300.0		407,300.0		407,100.0
U.S. mutual funds | Fixed income, including money market
Revenue and Services [Line Items]
Average assets under management	74,200.0	70,800.0	75,000.0	70,300.0
Period end assets under management	75,300.0		75,300.0		73,700.0
U.S. mutual funds | Multi-asset
Revenue and Services [Line Items]
Average assets under management	179,900.0	191,900.0	185,100.0	189,000.0
Period end assets under management	188,200.0		188,200.0		201,900.0
Subadvised and separate accounts and other investment products
Revenue and Services [Line Items]
Net revenues	470.7	409.5	922.3	787.8
Average assets under management	511,600.0	461,300.0	509,900.0	447,400.0
Period end assets under management	549,200.0		549,200.0		524,100.0
Subadvised and separate accounts and other investment products | Equity
Revenue and Services [Line Items]
Average assets under management	293,200.0	254,100.0	285,300.0	245,500.0
Period end assets under management	311,600.0		311,600.0		291,700.0
Subadvised and separate accounts and other investment products | Fixed income, including money market
Revenue and Services [Line Items]
Average assets under management	77,100.0	78,600.0	79,200.0	77,900.0
Period end assets under management	77,700.0		77,700.0		74,200.0
Subadvised and separate accounts and other investment products | Multi-asset
Revenue and Services [Line Items]
Average assets under management	141,300.0	128,600.0	145,400.0	124,000.0
Period end assets under management	159,900.0		159,900.0		$ 158,200.0
Other clients
Revenue and Services [Line Items]
Net revenues	26.5	25.2	56.2	51.5
Investment advisory fees
Revenue and Services [Line Items]
Net revenues	1,293.8	1,270.2	2,621.6	2,464.4
Investment advisory fees | U.S. mutual funds
Revenue and Services [Line Items]
Net revenues	823.1	860.7	1,699.3	1,676.6
Investment advisory fees | U.S. mutual funds | Equity
Revenue and Services [Line Items]
Net revenues	545.9	553.8	1,114.3	1,071.2
Investment advisory fees | U.S. mutual funds | Fixed income, including money market
Revenue and Services [Line Items]
Net revenues	66.8	69.1	138.5	137.9
Investment advisory fees | U.S. mutual funds | Multi-asset
Revenue and Services [Line Items]
Net revenues	210.4	237.8	446.5	467.5
Investment advisory fees | Subadvised and separate accounts and other investment products
Revenue and Services [Line Items]
Net revenues	470.7	409.5	922.3	787.8
Investment advisory fees | Subadvised and separate accounts and other investment products | Equity
Revenue and Services [Line Items]
Net revenues	305.6	255.0	585.1	491.5
Investment advisory fees | Subadvised and separate accounts and other investment products | Fixed income, including money market
Revenue and Services [Line Items]
Net revenues	33.9	39.0	71.9	76.3
Investment advisory fees | Subadvised and separate accounts and other investment products | Multi-asset
Revenue and Services [Line Items]
Net revenues	131.2	115.5	265.3	220.0
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	95.9	94.9	202.8	197.8
Administrative fees | U.S. mutual funds
Revenue and Services [Line Items]
Net revenues	69.4	69.7	146.6	146.3
Administrative fees | Subadvised and separate accounts 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	26.5	25.2	56.2	51.5
Distribution and servicing fees
Revenue and Services [Line Items]
Net revenues	25.7	30.1	53.6	60.3
Distribution and servicing fees | U.S. mutual funds
Revenue and Services [Line Items]
Net revenues	25.7	30.1	53.6	60.3
Distribution and servicing fees | Subadvised and separate accounts 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