Consolidated Investment Products (CIP) - Fair Value Hierarchy Levels Of Investments Held And Notes Issued By Consolidated Investment Products (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Segment Reporting, Other Significant Reconciling Items [Line Items]
Total assets at fair value	$ 8,304.8	$ 7,910.0
Bank loans
Segment Reporting, Other Significant Reconciling Items [Line Items]
Bank loan, bonds and equity securities	7,043.2
Consolidated
Segment Reporting, Other Significant Reconciling Items [Line Items]
Total assets at fair value	8,304.8	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	272.7	153.1
Consolidated | Significant Other Observable Inputs (Level 2)
Segment Reporting, Other Significant Reconciling Items [Line Items]
Total assets at fair value	7,728.6	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	303.5	258.2
Consolidated | Bank loans
Segment Reporting, Other Significant Reconciling Items [Line Items]
Bank loan, bonds and equity securities	7,053.8	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	7,053.8	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	562.2	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	0.0	0.6
Consolidated | Bonds | Significant Other Observable Inputs (Level 2)
Segment Reporting, Other Significant Reconciling Items [Line Items]
Bank loan, bonds and equity securities	562.2	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	343.7	137.2
Equity and fixed income mutual funds, and investments in other private equity funds	33.5	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	14.6	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	85.6	75.9
Equity and fixed income mutual funds, and investments in other private equity funds	18.9	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	311.6	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	$ 303.5	$ 258.2