Fair Value Measurements (Tables)

6 Months Ended

Jun. 30, 2013

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 June 30, 2013 and December 31, 2012, are as follows (in thousands):

	June 30, 2013
	Level 1		Level 2		Level 3		Total
United States treasuries	$	206,061	$	—	$	—	$	206,061
Commercial paper	—		4,000		—		4,000
Corporate bonds	—		75,628		—		75,628
Government agency securities	—		86,440		—		86,440
Total	$	206,061	$	166,068	$	—	$	372,129

	December 31, 2012
	Level 1		Level 2		Level 3		Total
United States treasuries	$	278,698	$	—	$	—	$	278,698
Government agency securities	—		123,054		—		123,054
Total	$	278,698	$	123,054	$	—	$	401,752