INFORMATION ABOUT RECEIVABLES, REVENUES, AND SERVICES. (Investment Advisory Services) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Net revenues	$ 1,732.9	$ 1,595.8	$ 1,415.4	$ 1,462.6	$ 1,468.7	$ 1,426.7	$ 1,395.2	$ 1,327.3	$ 6,206.7	$ 5,617.9	$ 5,372.6
Average assets under management									1,247,900.0	1,109,300.0	1,036,500.0
Period end assets under management	1,470,500.0				1,206,800.0				1,470,500.0	1,206,800.0
U.S. mutual funds
Disaggregation of Revenue [Line Items]
Net revenues									4,042.5	3,854.3	3,815.2
Average assets under management									687,700.0	640,400.0	621,800.0
Period end assets under management	794,600.0				682,700.0				794,600.0	682,700.0
U.S. mutual funds \| Equity and blended assets
Disaggregation of Revenue [Line Items]
Average assets under management									417,000.0	376,300.0	356,900.0
Period end assets under management	498,600.0				407,100.0				498,600.0	407,100.0
U.S. mutual funds \| Fixed income, including money market
Disaggregation of Revenue [Line Items]
Average assets under management									76,800.0	72,000.0	70,300.0
Period end assets under management	79,400.0				73,700.0				79,400.0	73,700.0
U.S. mutual funds \| Multi-asset
Disaggregation of Revenue [Line Items]
Average assets under management									193,900.0	192,100.0	194,600.0
Period end assets under management	216,600.0				201,900.0				216,600.0	201,900.0
Subadvised funds, separate accounts, collective investment trusts, and other investment products
Disaggregation of Revenue [Line Items]
Net revenues									2,053.2	1,660.0	1,475.6
Average assets under management									560,200.0	468,900.0	414,700.0
Period end assets under management	675,900.0				524,100.0				675,900.0	524,100.0
Subadvised funds, separate accounts, collective investment trusts, and other investment products \| Equity and blended assets
Disaggregation of Revenue [Line Items]
Average assets under management									321,300.0	257,300.0	218,100.0
Period end assets under management	397,200.0				291,700.0				397,200.0	291,700.0
Subadvised funds, separate accounts, collective investment trusts, and other investment products \| Fixed income, including money market
Disaggregation of Revenue [Line Items]
Average assets under management									82,100.0	79,500.0	92,600.0
Period end assets under management	89,300.0				74,200.0				89,300.0	74,200.0
Subadvised funds, separate accounts, collective investment trusts, and other investment products \| Multi-asset
Disaggregation of Revenue [Line Items]
Average assets under management									156,800.0	132,100.0	104,000.0
Period end assets under management	$ 189,400.0				$ 158,200.0				189,400.0	158,200.0
Other clients
Disaggregation of Revenue [Line Items]
Net revenues									111.0	103.6	81.8
Investment advisory fees
Disaggregation of Revenue [Line Items]
Net revenues									5,693.1	5,112.5	4,850.6
Investment advisory fees \| U.S. mutual funds
Disaggregation of Revenue [Line Items]
Net revenues									3,639.9	3,452.5	3,375.0
Investment advisory fees \| U.S. mutual funds \| Equity and blended assets
Disaggregation of Revenue [Line Items]
Net revenues									2,440.4	2,219.1	2,125.4
Investment advisory fees \| U.S. mutual funds \| Fixed income, including money market
Disaggregation of Revenue [Line Items]
Net revenues									266.5	280.1	288.2
Investment advisory fees \| U.S. mutual funds \| Multi-asset
Disaggregation of Revenue [Line Items]
Net revenues									933.0	953.3	961.4
Investment advisory fees \| Subadvised funds, separate accounts, collective investment trusts, and other investment products
Disaggregation of Revenue [Line Items]
Net revenues									2,053.2	1,660.0	1,475.6
Investment advisory fees \| Subadvised funds, separate accounts, collective investment trusts, and other investment products \| Equity and blended assets
Disaggregation of Revenue [Line Items]
Net revenues									1,326.3	1,033.4	892.0
Investment advisory fees \| Subadvised funds, separate accounts, collective investment trusts, and other investment products \| Fixed income, including money market
Disaggregation of Revenue [Line Items]
Net revenues									149.3	153.7	227.4
Investment advisory fees \| Subadvised funds, separate accounts, collective investment trusts, and other investment products \| Multi-asset
Disaggregation of Revenue [Line Items]
Net revenues									577.6	472.9	356.2
Investment advisory fees \| Other clients
Disaggregation of Revenue [Line Items]
Net revenues									0.0	0.0	0.0
Administrative fees
Disaggregation of Revenue [Line Items]
Net revenues									402.3	385.4	384.0
Administrative fees \| U.S. mutual funds
Disaggregation of Revenue [Line Items]
Net revenues									291.3	281.8	302.2
Administrative fees \| Subadvised funds, separate accounts, collective investment trusts, and other investment products
Disaggregation of Revenue [Line Items]
Net revenues									0.0	0.0	0.0
Administrative fees \| Other clients
Disaggregation of Revenue [Line Items]
Net revenues									111.0	103.6	81.8
Distribution and servicing fees
Disaggregation of Revenue [Line Items]
Net revenues									111.3	120.0	138.0
Distribution and servicing fees \| U.S. mutual funds
Disaggregation of Revenue [Line Items]
Net revenues									111.3	120.0	138.0
Distribution and servicing fees \| Subadvised funds, separate accounts, collective investment trusts, and other investment products
Disaggregation of Revenue [Line Items]
Net revenues									0.0	0.0	0.0
Distribution and servicing fees \| Other clients
Disaggregation of Revenue [Line Items]
Net revenues									$ 0.0	$ 0.0	$ 0.0