Fair Value Measurements (Details) (Fair value measured on a recurring basis, USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Money market funds
Fair value measurements
Investments	$ 272.9	[1]	$ 170.5	[1]
Estimated Fair Value
Fair value measurements
Deferred compensation liabilities	(29.9)	[2]	(44.3)	[2]
Interest Rate Derivative Liabilities, at Fair Value	(0.4)	[2]	(0.8)	[2]
Total	858.4		836.1
Estimated Fair Value | Equity securities
Fair value measurements
Investments	611.7	[3]	698.0	[3]
Estimated Fair Value | Pooled funds
Fair value measurements
Investments	4.1	[3]	12.4	[3]
Estimated Fair Value | Fixed income securities
Fair value measurements
Investments			0.3	[3]
Quoted prices in Active Markets for Identical Assets (Level 1)
Fair value measurements
Deferred compensation liabilities	(29.9)	[2]	(44.3)	[2]
Interest Rate Derivative Liabilities, at Fair Value	0	[2]	0	[2]
Total	854.7		824.2
Quoted prices in Active Markets for Identical Assets (Level 1) | Money market funds
Fair value measurements
Investments	272.9	[1]	170.5	[1]
Quoted prices in Active Markets for Identical Assets (Level 1) | Equity securities
Fair value measurements
Investments	611.7	[3]	698.0	[3]
Quoted prices in Active Markets for Identical Assets (Level 1) | Pooled funds
Fair value measurements
Investments	0	[3]	0	[3]
Quoted prices in Active Markets for Identical Assets (Level 1) | Fixed income securities
Fair value measurements
Investments			0	[3]
Significant Other Observable Inputs (Level 2)
Fair value measurements
Deferred compensation liabilities	0	[2]	0	[2]
Interest Rate Derivative Liabilities, at Fair Value	(0.4)	[2]	(0.8)	[2]
Total	3.7		11.9
Significant Other Observable Inputs (Level 2) | Money market funds
Fair value measurements
Investments	0	[1]	0	[1]
Significant Other Observable Inputs (Level 2) | Equity securities
Fair value measurements
Investments	0	[3]	0	[3]
Significant Other Observable Inputs (Level 2) | Pooled funds
Fair value measurements
Investments	4.1	[3]	12.4	[3]
Significant Other Observable Inputs (Level 2) | Fixed income securities
Fair value measurements
Investments			0.3	[3]
Fair Value, Inputs, Level 3 [Member]
Fair value measurements
Deferred compensation liabilities	0	[2]	0	[2]
Interest Rate Derivative Liabilities, at Fair Value	0	[2]	0	[2]
Total	0		0
Fair Value, Inputs, Level 3 [Member] | Money market funds
Fair value measurements
Investments	0	[1]	0	[1]
Fair Value, Inputs, Level 3 [Member] | Equity securities
Fair value measurements
Investments	0	[3]	0	[3]
Fair Value, Inputs, Level 3 [Member] | Pooled funds
Fair value measurements
Investments	0	[3]	0	[3]
Fair Value, Inputs, Level 3 [Member] | Fixed income securities
Fair value measurements
Investments			$ 0	[3]

[1] (1) Included in Cash and cash equivalents, Funds held on behalf of clients and Other assets on the Company's Condensed Consolidated Balance Sheet.

[2] (3) Included in Other liabilities on the Company's Condensed Consolidated Balance Sheet.

[3] (2) Included in Investments on the Company's Condensed Consolidated Balance Sheet.