Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Major categories of assets and liabilities measured on a recurring basis, at fair value
Derivative instrument	$ 0.1
Assets and liabilities measured on a recurring basis | Level 1
Major categories of assets and liabilities measured on a recurring basis, at fair value
Total	247.8	273.6
Assets and liabilities measured on a recurring basis | Level 1 | Treasury bills
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	86.6	70.2
Assets and liabilities measured on a recurring basis | 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	106.4	114.8
Assets and liabilities measured on a recurring basis | Level 1 | Government Agency Securities
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	54.8	88.6
Assets and liabilities measured on a recurring basis | Level 2
Major categories of assets and liabilities measured on a recurring basis, at fair value
Derivative instrument	0.1
Total	119.1	101.4
Assets and liabilities measured on a recurring basis | Level 2 | Treasury bills
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	51.0	20.0
Assets and liabilities measured on a recurring basis | 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	68.0	81.2
Assets and liabilities measured on a recurring basis | 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
Assets and liabilities measured on a recurring basis | Fair Value
Major categories of assets and liabilities measured on a recurring basis, at fair value
Derivative instrument	0.1
Total	366.9	375.0
Assets and liabilities measured on a recurring basis | Fair Value | Treasury bills
Major categories of assets and liabilities measured on a recurring basis, at fair value
Cash equivalents and short-term investments	137.6	90.2
Assets and liabilities measured on a recurring basis | 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	106.4	114.8
Assets and liabilities measured on a recurring basis | 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	122.8	169.8
Assets and liabilities measured on a recurring basis | 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