Fair Value Measurements (Details) (Assets and liabilities measured on a recurring basis, USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Level 1
Major categories of assets and liabilities measured on a recurring basis, at fair value
Total	$ 246.3	$ 90.2
Level 1 | Treasury bills
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	246.3	90.2
Level 2
Major categories of assets and liabilities measured on a recurring basis, at fair value
Total	146.1	284.8
Level 2 | Government agency securities
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	117.9	169.8
Level 2 | FDIC guaranteed corporate debt
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	28.2	114.8
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
Fair Value
Major categories of assets and liabilities measured on a recurring basis, at fair value
Total	392.4	375.0
Fair Value | Treasury bills
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	246.3	90.2
Fair Value | Government agency securities
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	117.9	169.8
Fair Value | FDIC guaranteed corporate debt
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	28.2	114.8
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