INFORMATION ABOUT RECEIVABLES, REVENUES, AND SERVICES - Investment Advisory Services (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021
Revenue and Services [Line Items]
Net revenues	$ 1,863.0	$ 1,826.8
Average assets under management	1,559,900.0	1,508,800.0
Period end assets under management	1,551,800.0		$ 1,687,800.0
U.S. mutual funds
Revenue and Services [Line Items]
Net revenues	1,095.9	1,158.0
Average assets under management	798,000.0	814,500.0
Period end assets under management	789,400.0		871,400.0
U.S. mutual funds \| Equity
Revenue and Services [Line Items]
Average assets under management	491,300.0	509,600.0
Period end assets under management	489,000.0		553,900.0
U.S. mutual funds \| Fixed income, including money market
Revenue and Services [Line Items]
Average assets under management	86,400.0	82,300.0
Period end assets under management	83,900.0		85,300.0
U.S. mutual funds \| Multi-asset
Revenue and Services [Line Items]
Average assets under management	220,300.0	222,600.0
Period end assets under management	216,500.0		232,200.0
Subadvised funds, separate accounts, collective investment trusts, and other investment products
Revenue and Services [Line Items]
Net revenues	730.0	637.6
Average assets under management	761,900.0	694,300.0
Period end assets under management	762,400.0		816,400.0
Subadvised funds, separate accounts, collective investment trusts, and other investment products \| Equity
Revenue and Services [Line Items]
Average assets under management	395,200.0	408,400.0
Period end assets under management	389,000.0		438,800.0
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	91,400.0	90,700.0
Period end assets under management	91,300.0		90,400.0
Subadvised funds, separate accounts, collective investment trusts, and other investment products \| Multi-asset
Revenue and Services [Line Items]
Average assets under management	233,400.0	195,200.0
Period end assets under management	239,900.0		245,500.0
Subadvised funds, separate accounts, collective investment trusts, and other investment products \| Alternatives
Revenue and Services [Line Items]
Average assets under management	41,900.0	0.0
Period end assets under management	42,200.0		$ 41,700.0
Other clients
Revenue and Services [Line Items]
Net revenues	37.1	31.2
Investment advisory fees
Revenue and Services [Line Items]
Net revenues	1,662.1	1,687.8
Investment advisory fees \| U.S. mutual funds
Revenue and Services [Line Items]
Net revenues	976.5	1,050.2
Investment advisory fees \| U.S. mutual funds \| Equity
Revenue and Services [Line Items]
Net revenues	691.9	729.8
Investment advisory fees \| U.S. mutual funds \| Fixed income, including money market
Revenue and Services [Line Items]
Net revenues	64.0	59.0
Investment advisory fees \| U.S. mutual funds \| Multi-asset
Revenue and Services [Line Items]
Net revenues	220.6	261.4
Investment advisory fees \| Subadvised funds, separate accounts, collective investment trusts, and other investment products
Revenue and Services [Line Items]
Net revenues	685.6	637.6
Investment advisory fees \| Subadvised funds, separate accounts, collective investment trusts, and other investment products \| Equity
Revenue and Services [Line Items]
Net revenues	394.1	424.2
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	42.6	37.5
Investment advisory fees \| Subadvised funds, separate accounts, collective investment trusts, and other investment products \| Multi-asset
Revenue and Services [Line Items]
Net revenues	184.0	175.9
Investment advisory fees \| Subadvised funds, separate accounts, collective investment trusts, and other investment products \| Alternatives
Revenue and Services [Line Items]
Net revenues	64.9	0.0
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	130.2	109.9
Administrative fees \| U.S. mutual funds
Revenue and Services [Line Items]
Net revenues	93.1	78.7
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	37.1	31.2
Distribution and servicing fees
Revenue and Services [Line Items]
Net revenues	26.3	29.1
Distribution and servicing fees \| U.S. mutual funds
Revenue and Services [Line Items]
Net revenues	26.3	29.1
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
Distribution and servicing fees \| Other clients
Revenue and Services [Line Items]
Net revenues	0.0	0.0
Capital allocation-based income
Revenue and Services [Line Items]
Net revenues	44.4	$ 0.0
Capital allocation-based income \| U.S. mutual funds
Revenue and Services [Line Items]
Net revenues	0.0
Capital allocation-based income \| Subadvised funds, separate accounts, collective investment trusts, and other investment products
Revenue and Services [Line Items]
Net revenues	44.4
Capital allocation-based income \| Other clients
Revenue and Services [Line Items]
Net revenues	$ 0.0