Fair Value Measurements (Details) (Assets and liabilities measured on a recurring basis, USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Level 1
Major categories of assets and liabilities measured on a recurring basis, at fair value
Total	$ 278.7	$ 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	278.7	90.2
Level 2
Major categories of assets and liabilities measured on a recurring basis, at fair value
Total	123.0	284.8
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		114.8
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	123.0	169.8
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	401.7	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	278.7	90.2
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		114.8
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	123.0	169.8
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