Fair Value Measurements (Details) - USD ($) $ in Millions		Dec. 31, 2016	Dec. 31, 2015
Fair value measurements [Line items]
Investments		$ 377.4	$ 418.2
Cost method, private equity and other investments
Fair value measurements [Line items]
Investments		16.8	16.0
Other investments
Fair value measurements [Line items]
Investments		196.9	198.7
Other investments | Cost method, private equity and other investments
Fair value measurements [Line items]
Investments		128.0	133.2
Investments in pooled funds
Fair value measurements [Line items]
Alternative Investments, Fair Value Disclosure		11.5	6.1
Fair value measured on a recurring basis
Fair value measurements [Line items]
Deferred compensation liabilities	[1]	(7.9)	(14.8)
Derivative Liability	[1]	0.4	0.3
Fair Value, Net Asset (Liability)		669.9	469.3
Fair value measured on a recurring basis | Money market funds
Fair value measurements [Line items]
Investments, Fair Value Disclosure	[2]	437.0	210.1
Fair value measured on a recurring basis | Equity securities
Fair value measurements [Line items]
Investments, Fair Value Disclosure	[3]	188.4	234.3
Fair value measured on a recurring basis | Seed Capital Investments
Fair value measurements [Line items]
Investments, Fair Value Disclosure	[3]	61.0	50.7
Fair value measured on a recurring basis | Securities sold short
Fair value measurements [Line items]
Investments, Fair Value Disclosure	[1]	(8.2)	(10.7)
Fair value measured on a recurring basis | Quoted prices in Active Markets for Identical Assets (Level 1)
Fair value measurements [Line items]
Deferred compensation liabilities	[1]	(7.9)	(14.8)
Derivative Liability	[1]	0.0	0.0
Fair Value, Net Asset (Liability)		670.3	469.6
Fair value measured on a recurring basis | Quoted prices in Active Markets for Identical Assets (Level 1) | Money market funds
Fair value measurements [Line items]
Investments, Fair Value Disclosure	[2]	437.0	210.1
Fair value measured on a recurring basis | Quoted prices in Active Markets for Identical Assets (Level 1) | Equity securities
Fair value measurements [Line items]
Investments, Fair Value Disclosure	[3]	188.4	234.3
Fair value measured on a recurring basis | Quoted prices in Active Markets for Identical Assets (Level 1) | Seed Capital Investments
Fair value measurements [Line items]
Investments, Fair Value Disclosure	[3]	61.0	50.7
Fair value measured on a recurring basis | Quoted prices in Active Markets for Identical Assets (Level 1) | Securities sold short
Fair value measurements [Line items]
Investments, Fair Value Disclosure	[1]	(8.2)	(10.7)
Fair value measured on a recurring basis | Significant Other Observable Inputs (Level 2)
Fair value measurements [Line items]
Deferred compensation liabilities	[1]	0.0	0.0
Derivative Liability	[1]	0.4	0.3
Fair Value, Net Asset (Liability)		(0.4)	(0.3)
Fair value measured on a recurring basis | Significant Other Observable Inputs (Level 2) | Money market funds
Fair value measurements [Line items]
Investments, Fair Value Disclosure	[2]	0.0	0.0
Fair value measured on a recurring basis | Significant Other Observable Inputs (Level 2) | Equity securities
Fair value measurements [Line items]
Investments, Fair Value Disclosure	[3]	0.0	0.0
Fair value measured on a recurring basis | Significant Other Observable Inputs (Level 2) | Seed Capital Investments
Fair value measurements [Line items]
Investments, Fair Value Disclosure	[3]	0.0	0.0
Fair value measured on a recurring basis | Significant Other Observable Inputs (Level 2) | Securities sold short
Fair value measurements [Line items]
Investments, Fair Value Disclosure	[1]	0.0	0.0
Fair value measured on a recurring basis | Significant Unobservable Inputs (Level 3)
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
Fair value measured on a recurring basis | Significant Unobservable Inputs (Level 3) | Money market funds
Fair value measurements [Line items]
Investments, Fair Value Disclosure	[2]	0.0	0.0
Fair value measured on a recurring basis | Significant Unobservable Inputs (Level 3) | Equity securities
Fair value measurements [Line items]
Investments, Fair Value Disclosure	[3]	0.0	0.0
Fair value measured on a recurring basis | Significant Unobservable Inputs (Level 3) | Seed Capital Investments
Fair value measurements [Line items]
Investments, Fair Value Disclosure	[3]	0.0	0.0
Fair value measured on a recurring basis | Significant Unobservable Inputs (Level 3) | Securities sold short
Fair value measurements [Line items]
Investments, Fair Value Disclosure	[1]	$ 0.0	$ 0.0

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

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

[3] Included in Investments on our Consolidated Balance Sheet.