Fair Value Measurements (Details) (USD $) In Millions	Jun. 30, 2011	Dec. 31, 2010
Major categories of assets and liabilities measured on a recurring basis, at fair value
Derivative instrument		$ 0.3
Level 1 | Assets and liabilities measured on a recurring basis
Major categories of assets and liabilities measured on a recurring basis, at fair value
Treasury bills	149.6	136.2
FDIC insured corporate bonds	128.0	129.4
Commercial paper	102.9	128.6
Total	380.5	394.2
Level 2 | Assets and liabilities measured on a recurring basis
Major categories of assets and liabilities measured on a recurring basis, at fair value
Treasury bills		79.5
Commercial paper	52.5	62.8
Money market instruments	2.8	0.6
Derivative instrument		0.3
Total	55.3	143.2
Total | Assets and liabilities measured on a recurring basis
Major categories of assets and liabilities measured on a recurring basis, at fair value
Treasury bills	149.6	215.7
FDIC insured corporate bonds	128.0	129.4
Commercial paper	155.4	191.4
Money market instruments	2.8	0.6
Derivative instrument		0.3
Total	$ 435.8	$ 537.4