Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2012

Fair Value Measurements

Schedule of assets and liabilities measured on a recurring basis, at fair value

The major categories of assets and liabilities measured on a recurring basis, at fair value, as of December 31, 2012 and 2011 are as follows (in millions):

	December 31, 2012
	Level 1		Level 2		Level 3		Total
Treasury bills	$	278.7	$	—	$	—	$	278.7
Government agency securities		—		123.0		—		123.0
Total	$	278.7	$	123.0	$	—	$	401.7

	December 31, 2011
	Level 1		Level 2		Level 3		Total
Treasury bills	$	90.2	$	—	$	—	$	90.2
FDIC guaranteed corporate debt		—		114.8		—		114.8
Government agency securities		—		169.8		—		169.8
Money market instruments		—		0.2		—		0.2
Total	$	90.2	$	284.8	$	—	$	375.0

The classification in the fair value table as of December 31, 2011 has been revised to conform to current period classifications due to an immaterial error related to previously disclosed fair value hierarchy tables.