Fair Value Of Assets And Liabilities (Tri-Level Hierarchy, Carrying Value) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets held for policyholders	$ 10,442.4	$ 11,384.8
Total	11,519.1	12,035.6
Total	(49.9)	(40.9)
Contingent consideration liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration liability	(49.9)	(40.9)
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	610.4	367.6
Seed money
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments:	253.5	202.8
Investments related to deferred compensation plans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments:	209.7	78.6
Other equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments:	3.1	1.8
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets held for policyholders	10,442.4	11,384.8
Total	11,519.1	12,035.6
Total	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Contingent consideration liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration liability	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	610.4	367.6
Quoted Prices in Active Markets for Identical Assets (Level 1) | Seed money
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments:	253.5	202.8
Quoted Prices in Active Markets for Identical Assets (Level 1) | Investments related to deferred compensation plans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments:	209.7	78.6
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments:	3.1	1.8
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets held for policyholders	0.0	0.0
Total	0.0	0.0
Total	0.0	0.0
Significant Other Observable Inputs (Level 2) | Contingent consideration liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration liability	0.0	0.0
Significant Other Observable Inputs (Level 2) | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	0.0	0.0
Significant Other Observable Inputs (Level 2) | Seed money
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments:	0.0	0.0
Significant Other Observable Inputs (Level 2) | Investments related to deferred compensation plans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments:	0.0	0.0
Significant Other Observable Inputs (Level 2) | Other equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments:	0.0	0.0
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets held for policyholders	0.0	0.0
Total	0.0	0.0
Total	(49.9)	(40.9)
Significant Unobservable Inputs (Level 3) | Contingent consideration liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration liability	(49.9)	(40.9)
Significant Unobservable Inputs (Level 3) | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	0.0	0.0
Significant Unobservable Inputs (Level 3) | Seed money
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments:	0.0	0.0
Significant Unobservable Inputs (Level 3) | Investments related to deferred compensation plans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments:	0.0	0.0
Significant Unobservable Inputs (Level 3) | Other equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments:	$ 0.0	$ 0.0