Consolidated Investment Products (Fair Value Hierarchy Levels Of Investments Held And Notes Issued By Consolidated Investment Products) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Statement [Line Items]
Real estate investments	$ 32.0	$ 11.8
Total assets at fair value	7,041.7	6,213.5
Quoted Prices in Active Markets for Identical Assets (Level 1)
Statement [Line Items]
Real estate investments	0.0	0.0
Total assets at fair value	220.0	226.9
Significant Other Observable Inputs (Level 2)
Statement [Line Items]
Real estate investments	0.0	0.0
Total assets at fair value	6,581.1	5,786.1
Significant Unobservable Inputs (Level 3)
Statement [Line Items]
Real estate investments	32.0	11.8
Total assets at fair value	32.0	11.8
Investments Measured at NAV as a practical expedient
Statement [Line Items]
Real estate investments	0.0	0.0
Total assets at fair value	208.6	188.7
Bank loans
Statement [Line Items]
Bank loan, bonds and equity securities	5,801.7	5,117.0
Bank loans | Quoted Prices in Active Markets for Identical Assets (Level 1)
Statement [Line Items]
Bank loan, bonds and equity securities	0.0	0.0
Bank loans | Significant Other Observable Inputs (Level 2)
Statement [Line Items]
Bank loan, bonds and equity securities	5,801.7	5,117.0
Bank loans | Significant Unobservable Inputs (Level 3)
Statement [Line Items]
Bank loan, bonds and equity securities	0.0	0.0
Bank loans | Investments Measured at NAV as a practical expedient
Statement [Line Items]
Bank loan, bonds and equity securities	0.0	0.0
Bonds
Statement [Line Items]
Bank loan, bonds and equity securities	706.6	636.0
Bonds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Statement [Line Items]
Bank loan, bonds and equity securities	3.7	0.0
Bonds | Significant Other Observable Inputs (Level 2)
Statement [Line Items]
Bank loan, bonds and equity securities	702.9	636.0
Bonds | Significant Unobservable Inputs (Level 3)
Statement [Line Items]
Bank loan, bonds and equity securities	0.0	0.0
Bonds | Investments Measured at NAV as a practical expedient
Statement [Line Items]
Bank loan, bonds and equity securities	0.0	0.0
Equity securities, Equity and fixed income mutual funds
Statement [Line Items]
Bank loan, bonds and equity securities	271.4	241.2
Equity and fixed income mutual funds, and investments in other private equity funds	21.4	18.8
Equity securities, Equity and fixed income mutual funds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Statement [Line Items]
Bank loan, bonds and equity securities	194.9	208.1
Equity and fixed income mutual funds, and investments in other private equity funds	21.4	18.8
Equity securities, Equity and fixed income mutual funds | Significant Other Observable Inputs (Level 2)
Statement [Line Items]
Bank loan, bonds and equity securities	76.5	33.1
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 | Significant Unobservable Inputs (Level 3)
Statement [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
Statement [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
Statement [Line Items]
Equity and fixed income mutual funds, and investments in other private equity funds	208.6	188.7
Investments in other private equity funds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Statement [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)
Statement [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 Unobservable Inputs (Level 3)
Statement [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
Statement [Line Items]
Equity and fixed income mutual funds, and investments in other private equity funds	$ 208.6	$ 188.7