Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 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 March 31, 2013 and December 31, 2012, are as follows (in thousands):

	March 31, 2013
	Level 1		Level 2		Level 3		Total
Treasury bills	$	271,201	$	—	$	—	$	271,201
Government agency securities	—		48,054		—		48,054
Corporate bonds	—		63,499		—		63,499
Commercial paper	—		19,992		—		19,992
Total	$	271,201	$	131,545	$	—	$	402,746

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