Fair Value Measurements (Details) - USD ($) $ in Millions		Sep. 30, 2017	Dec. 31, 2016
Fair value measurements [Line Items]
Investments		$ 199.6	$ 377.4
Cost-method and other investments
Fair value measurements [Line Items]
Investments		48.4	16.8
Other investments
Fair value measurements [Line Items]
Investments		187.7	196.9
Other investments | Cost-method and other investments
Fair value measurements [Line Items]
Investments		141.9	128.0
Pooled funds
Fair value measurements [Line Items]
Investments in pooled funds		7.6	11.5
Fair value measured on a recurring basis
Fair value measurements [Line Items]
Deferred compensation liabilities	[1]	(34.1)	(7.9)
Derivative Liability	[1]	(0.4)	(0.4)
Fair Value, Net Asset (Liability)		283.4	669.9
Fair value measured on a recurring basis | Money market funds
Fair value measurements [Line Items]
Investments	[2]	260.2	437.0
Fair value measured on a recurring basis | Equity securities
Fair value measurements [Line Items]
Investments	[3]	46.0	188.4
Fair value measured on a recurring basis | Seed Capital Investments
Fair value measurements [Line Items]
Investments	[3]	11.7	61.0
Fair value measured on a recurring basis | Securities sold short
Fair value measurements [Line Items]
Investments	[1]		(8.2)
Quoted prices in Active Markets for Identical Assets (Level 1) | Fair value measured on a recurring basis
Fair value measurements [Line Items]
Deferred compensation liabilities	[1]	(34.1)	(7.9)
Derivative Liability	[1]	0.0	0.0
Fair Value, Net Asset (Liability)		283.8	670.3
Quoted prices in Active Markets for Identical Assets (Level 1) | Fair value measured on a recurring basis | Money market funds
Fair value measurements [Line Items]
Investments	[2]	260.2	437.0
Quoted prices in Active Markets for Identical Assets (Level 1) | Fair value measured on a recurring basis | Equity securities
Fair value measurements [Line Items]
Investments	[3]	46.0	188.4
Quoted prices in Active Markets for Identical Assets (Level 1) | Fair value measured on a recurring basis | Seed Capital Investments
Fair value measurements [Line Items]
Investments	[3]	11.7	61.0
Quoted prices in Active Markets for Identical Assets (Level 1) | Fair value measured on a recurring basis | Securities sold short
Fair value measurements [Line Items]
Investments	[1]		(8.2)
Significant Other Observable Inputs (Level 2) | Fair value measured on a recurring basis
Fair value measurements [Line Items]
Deferred compensation liabilities	[1]	0.0	0.0
Derivative Liability	[1]	(0.4)	(0.4)
Fair Value, Net Asset (Liability)		(0.4)	(0.4)
Significant Other Observable Inputs (Level 2) | Fair value measured on a recurring basis | Money market funds
Fair value measurements [Line Items]
Investments	[2]	0.0	0.0
Significant Other Observable Inputs (Level 2) | Fair value measured on a recurring basis | Equity securities
Fair value measurements [Line Items]
Investments	[3]	0.0	0.0
Significant Other Observable Inputs (Level 2) | Fair value measured on a recurring basis | Seed Capital Investments
Fair value measurements [Line Items]
Investments	[3]	0.0	0.0
Significant Other Observable Inputs (Level 2) | Fair value measured on a recurring basis | Securities sold short
Fair value measurements [Line Items]
Investments	[1]		0.0
Significant Unobservable Inputs (Level 3) | Fair value measured on a recurring basis
Fair value measurements [Line Items]
Deferred compensation liabilities	[1]	0.0	0.0
Derivative Liability	[1]	0.0	0.0
Fair Value, Net Asset (Liability)		0.0	0.0
Significant Unobservable Inputs (Level 3) | Fair value measured on a recurring basis | Money market funds
Fair value measurements [Line Items]
Investments	[2]	0.0	0.0
Significant Unobservable Inputs (Level 3) | Fair value measured on a recurring basis | Equity securities
Fair value measurements [Line Items]
Investments	[3]	0.0	0.0
Significant Unobservable Inputs (Level 3) | Fair value measured on a recurring basis | Seed Capital Investments
Fair value measurements [Line Items]
Investments	[3]	$ 0.0	0.0
Significant Unobservable Inputs (Level 3) | Fair value measured on a recurring basis | Securities sold short
Fair value measurements [Line Items]
Investments	[1]		$ 0.0

[1] Included in Other liabilities on the Condensed Consolidated Balance Sheet.

[2] Included in Cash and cash equivalents, Funds held on behalf of clients, and Other current assets on the Condensed Consolidated Balance Sheet.

[3] Included in Investments on the Condensed Consolidated Balance Sheet.