Consolidated Investment Products (CIP) - Fair Value Hierarchy Levels Of Investments Held And Notes Issued By Consolidated Investment Products (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Segment Reporting, Other Significant Reconciling Items [Line Items]
Total assets at fair value	$ 7,912.1	$ 7,910.0
Bank loans
Segment Reporting, Other Significant Reconciling Items [Line Items]
Bank loan, bonds and equity securities	6,679.6
Consolidated
Segment Reporting, Other Significant Reconciling Items [Line Items]
Total assets at fair value	7,912.1	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	272.6	153.1
Consolidated | Significant Other Observable Inputs (Level 2)
Segment Reporting, Other Significant Reconciling Items [Line Items]
Total assets at fair value	7,384.1	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	255.4	258.2
Consolidated | Bank loans
Segment Reporting, Other Significant Reconciling Items [Line Items]
Bank loan, bonds and equity securities	6,696.4	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	6,696.4	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	570.2	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	0.5	0.6
Consolidated | Bonds | Significant Other Observable Inputs (Level 2)
Segment Reporting, Other Significant Reconciling Items [Line Items]
Bank loan, bonds and equity securities	569.7	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	353.3	137.2
Equity and fixed income mutual funds, and investments in other private equity funds	28.7	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	258.1	61.3
Equity and fixed income mutual funds, and investments in other private equity funds	14.0	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	95.2	75.9
Equity and fixed income mutual funds, and investments in other private equity funds	14.7	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	263.5	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	$ 255.4	$ 258.2