Fair Value Measurements (Details) (Assets and liabilities measured on a recurring basis, USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Level 1
Major categories of assets and liabilities measured on a recurring basis, at fair value
Total	$ 273.6	$ 394.2
Level 1 | Treasury bills
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	70.2	136.2
Level 1 | FDIC insured corporate bonds
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	187.5	129.4
Level 1 | Commercial paper
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	15.9	128.6
Level 2
Major categories of assets and liabilities measured on a recurring basis, at fair value
Derivative instrument		0.3
Total	101.4	143.2
Level 2 | Treasury bills
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	20.0	79.5
Level 2 | Commercial paper
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	81.2	62.8
Level 2 | Money market instruments
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	0.2	0.6
Fair Value
Major categories of assets and liabilities measured on a recurring basis, at fair value
Derivative instrument		0.3
Total	375.0	537.4
Fair Value | Treasury bills
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	90.2	215.7
Fair Value | FDIC insured corporate bonds
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	187.5	129.4
Fair Value | Commercial paper
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	97.1	191.4
Fair Value | Money market instruments
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	$ 0.2	$ 0.6