Consolidated Investment Products (Fair Value Hierarchy Levels Of Investments Held And Notes Issued By Consolidated Investment Products) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Real Estate Investments, Net	$ 81.2	$ 76.2
Investments Of Consolidated Investment Products	5,453.5	5,658.0
Fair Value, Inputs, Level 1 [Member]
Real Estate Investments, Net	0.0	0.0
Investments Of Consolidated Investment Products	215.7	217.8
Significant Other Observable Inputs (Level 2) [Member]
Real Estate Investments, Net	0.0	0.0
Investments Of Consolidated Investment Products	4,991.5	5,200.6
Significant Unobservable Inputs (Level 3) [Member]
Real Estate Investments, Net	81.2	76.2
Investments Of Consolidated Investment Products	81.2	76.2
Bank Loans [Member]
CLO Collateral Assets	4,693.5	4,894.2
Bank Loans [Member] | Fair Value, Inputs, Level 1 [Member]
CLO Collateral Assets	0.0	0.0
Bank Loans [Member] | Significant Other Observable Inputs (Level 2) [Member]
CLO Collateral Assets	4,693.5	4,894.2
Bank Loans [Member] | Significant Unobservable Inputs (Level 3) [Member]
CLO Collateral Assets	0.0	0.0
Bonds [Member]
CLO Collateral Assets	296.3	302.0
Bonds [Member] | Fair Value, Inputs, Level 1 [Member]
CLO Collateral Assets	0.0	0.0
Bonds [Member] | Significant Other Observable Inputs (Level 2) [Member]
CLO Collateral Assets	296.3	302.0
Bonds [Member] | Significant Unobservable Inputs (Level 3) [Member]
CLO Collateral Assets	0.0	0.0
Equity Securities [Member]
CLO Collateral Assets	198.2	203.2
Private Equity Fund Assets	19.2	19.0
Equity Securities [Member] | Fair Value, Inputs, Level 1 [Member]
CLO Collateral Assets	196.5	198.8
Private Equity Fund Assets	19.2	19.0
Equity Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
CLO Collateral Assets	1.7	4.4
Private Equity Fund Assets	0.0	0.0
Equity Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
CLO Collateral Assets	0.0	0.0
Private Equity Fund Assets	0.0	0.0
Private Equity Funds [Member]
Private Equity Fund Assets	165.1	163.4
Private Equity Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Private Equity Fund Assets	0.0	0.0
Private Equity Funds [Member] | Significant Other Observable Inputs (Level 2) [Member]
Private Equity Fund Assets	0.0	0.0
Private Equity Funds [Member] | Significant Unobservable Inputs (Level 3) [Member]
Private Equity Fund Assets	0.0	0.0
Investments Measured at NAV [Member]
Real Estate Investments, Net	0.0	0.0
Investments Of Consolidated Investment Products	165.1	163.4
Investments Measured at NAV [Member] | Bank Loans [Member]
CLO Collateral Assets	0.0	0.0
Investments Measured at NAV [Member] | Bonds [Member]
CLO Collateral Assets	0.0	0.0
Investments Measured at NAV [Member] | Equity Securities [Member]
CLO Collateral Assets	0.0	0.0
Private Equity Fund Assets	0.0	0.0
Investments Measured at NAV [Member] | Private Equity Funds [Member]
Private Equity Fund Assets	$ 165.1	$ 163.4