Consolidated Investment Products (CIP) - Fair Value Hierarchy Levels Of Investments Held And Notes Issued By Consolidated Investment Products (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Segment Reporting, Other Significant Reconciling Items [Line Items]
Real estate investments	$ 89.4	$ 78.6
Total assets at fair value	7,563.9	7,808.0
Quoted Prices in Active Markets for Identical Assets (Level 1)
Segment Reporting, Other Significant Reconciling Items [Line Items]
Real estate investments	0.0	0.0
Total assets at fair value	142.8	225.2
Significant Other Observable Inputs (Level 2)
Segment Reporting, Other Significant Reconciling Items [Line Items]
Real estate investments	0.0	0.0
Total assets at fair value	7,119.9	7,290.8
Significant Unobservable Inputs (Level 3)
Segment Reporting, Other Significant Reconciling Items [Line Items]
Real estate investments	89.4	78.6
Total assets at fair value	89.4	78.6
Investments Measured at NAV as a practical expedient
Segment Reporting, Other Significant Reconciling Items [Line Items]
Real estate investments	0.0	0.0
Total assets at fair value	211.8	213.4
Bank loans
Segment Reporting, Other Significant Reconciling Items [Line Items]
Bank loan, bonds and equity securities	6,467.6	6,504.4
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
Bank loans | Significant Other Observable Inputs (Level 2)
Segment Reporting, Other Significant Reconciling Items [Line Items]
Bank loan, bonds and equity securities	6,467.6	6,504.4
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
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
Bonds
Segment Reporting, Other Significant Reconciling Items [Line Items]
Bank loan, bonds and equity securities	552.3	705.9
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	0.4	0.5
Bonds | Significant Other Observable Inputs (Level 2)
Segment Reporting, Other Significant Reconciling Items [Line Items]
Bank loan, bonds and equity securities	551.9	705.4
Bonds | Significant Unobservable Inputs (Level 3)
Segment Reporting, Other Significant Reconciling Items [Line Items]
Bank loan, bonds and equity securities	0.0	0.0
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
Equity securities, Equity and fixed income mutual funds
Segment Reporting, Other Significant Reconciling Items [Line Items]
Bank loan, bonds and equity securities	206.8	275.9
Equity and fixed income mutual funds, and investments in other private equity funds	27.0	29.8
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	124.9	204.4
Equity and fixed income mutual funds, and investments in other private equity funds	17.5	20.3
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	81.9	71.5
Equity and fixed income mutual funds, and investments in other private equity funds	9.5	9.5
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
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
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	220.8	213.4
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
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	9.0	0.0
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
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	$ 211.8	$ 213.4