CONSOLIDATED INVESTMENT PRODUCTS (CIP) - Fair Value Hierarchy Levels of Investments Held and Notes Issued by Consolidated Investment Products (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Segment Reporting, Other Significant Reconciling Items [Line Items]
Total assets at fair value	$ 8,924.2	$ 7,910.0
Bank loans
Segment Reporting, Other Significant Reconciling Items [Line Items]
Bank loan, bonds and equity securities	7,225.4
Consolidated
Segment Reporting, Other Significant Reconciling Items [Line Items]
Total assets at fair value	8,924.2	7,910.0
Consolidated | Quoted Prices in Active Markets for Identical Assets (Level 1)
Segment Reporting, Other Significant Reconciling Items [Line Items]
Total assets at fair value	159.5	153.1
Consolidated | Significant Other Observable Inputs (Level 2)
Segment Reporting, Other Significant Reconciling Items [Line Items]
Total assets at fair value	8,391.2	7,498.7
Consolidated | Significant Unobservable Inputs (Level 3)
Segment Reporting, Other Significant Reconciling Items [Line Items]
Total assets at fair value	0.0	0.0
Consolidated | Investments Measured at NAV as a practical expedient
Segment Reporting, Other Significant Reconciling Items [Line Items]
Total assets at fair value	373.5	258.2
Consolidated | Bank loans
Segment Reporting, Other Significant Reconciling Items [Line Items]
Bank loan, bonds and equity securities	7,239.3	6,864.5
Consolidated | Bank loans | Quoted Prices in Active Markets for Identical Assets (Level 1)
Segment Reporting, Other Significant Reconciling Items [Line Items]
Bank loan, bonds and equity securities	0.0	0.0
Consolidated | Bank loans | Significant Other Observable Inputs (Level 2)
Segment Reporting, Other Significant Reconciling Items [Line Items]
Bank loan, bonds and equity securities	7,239.3	6,864.5
Consolidated | Bank loans | Significant Unobservable Inputs (Level 3)
Segment Reporting, Other Significant Reconciling Items [Line Items]
Bank loan, bonds and equity securities	0.0	0.0
Consolidated | Bank loans | Investments Measured at NAV as a practical expedient
Segment Reporting, Other Significant Reconciling Items [Line Items]
Bank loan, bonds and equity securities	0.0	0.0
Consolidated | Bonds
Segment Reporting, Other Significant Reconciling Items [Line Items]
Bank loan, bonds and equity securities	667.3	539.0
Consolidated | Bonds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Segment Reporting, Other Significant Reconciling Items [Line Items]
Bank loan, bonds and equity securities	10.1	0.6
Consolidated | Bonds | Significant Other Observable Inputs (Level 2)
Segment Reporting, Other Significant Reconciling Items [Line Items]
Bank loan, bonds and equity securities	657.2	538.4
Consolidated | Bonds | Significant Unobservable Inputs (Level 3)
Segment Reporting, Other Significant Reconciling Items [Line Items]
Bank loan, bonds and equity securities	0.0	0.0
Consolidated | Bonds | Investments Measured at NAV as a practical expedient
Segment Reporting, Other Significant Reconciling Items [Line Items]
Bank loan, bonds and equity securities	0.0	0.0
Consolidated | Equity securities, Equity and fixed income mutual funds
Segment Reporting, Other Significant Reconciling Items [Line Items]
Bank loan, bonds and equity securities	367.1	137.2
Equity and fixed income mutual funds, and investments in other private equity funds	268.9	103.0
Consolidated | Equity securities, Equity and fixed income mutual funds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Segment Reporting, Other Significant Reconciling Items [Line Items]
Bank loan, bonds and equity securities	149.2	61.3
Equity and fixed income mutual funds, and investments in other private equity funds	0.2	91.2
Consolidated | Equity securities, Equity and fixed income mutual funds | Significant Other Observable Inputs (Level 2)
Segment Reporting, Other Significant Reconciling Items [Line Items]
Bank loan, bonds and equity securities	217.9	75.9
Equity and fixed income mutual funds, and investments in other private equity funds	268.7	11.8
Consolidated | Equity securities, Equity and fixed income mutual funds | Significant Unobservable Inputs (Level 3)
Segment Reporting, Other Significant Reconciling Items [Line Items]
Bank loan, bonds and equity securities	0.0	0.0
Equity and fixed income mutual funds, and investments in other private equity funds	0.0	0.0
Consolidated | Equity securities, Equity and fixed income mutual funds | Investments Measured at NAV as a practical expedient
Segment Reporting, Other Significant Reconciling Items [Line Items]
Bank loan, bonds and equity securities	0.0	0.0
Equity and fixed income mutual funds, and investments in other private equity funds	0.0	0.0
Consolidated | Investments in other private equity funds
Segment Reporting, Other Significant Reconciling Items [Line Items]
Equity and fixed income mutual funds, and investments in other private equity funds	381.6	266.3
Consolidated | Investments in other private equity funds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Segment Reporting, Other Significant Reconciling Items [Line Items]
Equity and fixed income mutual funds, and investments in other private equity funds	0.0	0.0
Consolidated | Investments in other private equity funds | Significant Other Observable Inputs (Level 2)
Segment Reporting, Other Significant Reconciling Items [Line Items]
Equity and fixed income mutual funds, and investments in other private equity funds	8.1	8.1
Consolidated | Investments in other private equity funds | Significant Unobservable Inputs (Level 3)
Segment Reporting, Other Significant Reconciling Items [Line Items]
Equity and fixed income mutual funds, and investments in other private equity funds	0.0	0.0
Consolidated | Investments in other private equity funds | Investments Measured at NAV as a practical expedient
Segment Reporting, Other Significant Reconciling Items [Line Items]
Equity and fixed income mutual funds, and investments in other private equity funds	$ 373.5	$ 258.2