Fair Value Measurements (Details) - USD ($) $ in Millions		Jun. 30, 2016	Dec. 31, 2015
Fair value measurements
Investments		$ 333.6	$ 418.2
Cost-method and other investments
Fair value measurements
Investments		16.8	16.0
Other investments
Fair value measurements
Investments		204.2	198.7
Other investments | Cost-method and other investments
Fair value measurements
Investments		126.2	133.2
Pooled funds
Fair value measurements
Investments in pooled funds		7.0	6.1
Fair value measured on a recurring basis
Fair value measurements
Derivative Liability	[1]		0.3
Derivative Asset	[1]	0.5
Deferred compensation liabilities	[1]	(13.1)	(14.8)
Fair Value, Net Asset (Liability)		373.5	469.3
Fair value measured on a recurring basis | Money market funds
Fair value measurements
Investments	[2]	192.5	210.1
Fair value measured on a recurring basis | Equity securities
Fair value measurements
Investments	[3]	142.5	234.3
Fair value measured on a recurring basis | Seed Capital Investments
Fair value measurements
Investments	[3]	64.9	50.7
Fair value measured on a recurring basis | Securities sold short
Fair value measurements
Investments	[1]	(13.8)	(10.7)
Quoted prices in Active Markets for Identical Assets (Level 1) | Fair value measured on a recurring basis
Fair value measurements
Derivative Liability	[1]		0.0
Derivative Asset	[1]	0.0
Deferred compensation liabilities	[1]	(13.1)	(14.8)
Fair Value, Net Asset (Liability)		373.0	469.6
Quoted prices in Active Markets for Identical Assets (Level 1) | Fair value measured on a recurring basis | Money market funds
Fair value measurements
Investments	[2]	192.5	210.1
Quoted prices in Active Markets for Identical Assets (Level 1) | Fair value measured on a recurring basis | Equity securities
Fair value measurements
Investments	[3]	142.5	234.3
Quoted prices in Active Markets for Identical Assets (Level 1) | Fair value measured on a recurring basis | Seed Capital Investments
Fair value measurements
Investments	[3]	64.9	50.7
Quoted prices in Active Markets for Identical Assets (Level 1) | Fair value measured on a recurring basis | Securities sold short
Fair value measurements
Investments	[1]	(13.8)	(10.7)
Significant Other Observable Inputs (Level 2) | Fair value measured on a recurring basis
Fair value measurements
Derivative Liability	[1]		0.3
Derivative Asset	[1]	0.5
Deferred compensation liabilities	[1]	0.0	0.0
Fair Value, Net Asset (Liability)		0.5	(0.3)
Significant Other Observable Inputs (Level 2) | Fair value measured on a recurring basis | Money market funds
Fair value measurements
Investments	[2]	0.0	0.0
Significant Other Observable Inputs (Level 2) | Fair value measured on a recurring basis | Equity securities
Fair value measurements
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
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
Investments	[1]	0.0	0.0
Significant Unobservable Inputs (Level 3) | Fair value measured on a recurring basis
Fair value measurements
Derivative Liability	[1]		0.0
Derivative Asset	[1]	0.0
Deferred compensation liabilities	[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
Investments	[2]	0.0	0.0
Significant Unobservable Inputs (Level 3) | Fair value measured on a recurring basis | Equity securities
Fair value measurements
Investments	[3]	0.0	0.0
Significant Unobservable Inputs (Level 3) | Fair value measured on a recurring basis | Seed Capital Investments
Fair value measurements
Investments	[3]	0.0	0.0
Significant Unobservable Inputs (Level 3) | Fair value measured on a recurring basis | Securities sold short
Fair value measurements
Investments	[1]	$ 0.0	$ 0.0

[1] Included in Other assets or 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.