Fair Value of Financial Instruments (Tables)

6 Months Ended

Jun. 30, 2011

Fair Value Disclosures [Abstract]

Schedule of Fair Value - Assets Measured on Recurring Basis

The following tables set forth by level within the fair value hierarchy the Company's assets and liabilities that were recorded at fair value as of June 30, 2011 and December 31, 2010 (in thousands).

	At Fair Value as of June 30, 2011
	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	—	$	231,417	$	—	$	231,417
Commercial paper	—		181,886		—		181,886
U.S. government or government agency securities	—		265,261		—		265,261
Total	$	—	$	678,564	$	—	$	678,564

	At Fair Value as of December 31, 2010
	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	—	$	168,831	$	—	$	168,831
Commercial paper	—		17,494		—		17,494
U.S. government or government agency securities	—		108,364		—		108,364
Total	$	—	$	294,689	$	—	$	294,689

Available-for-sale Securities

Available-for-sale securities were comprised as follows as of June 30, 2011 and December 31, 2010 (in thousands):

	As of June 30, 2011
	Cost		Fair Value
Money market funds	$	231,417	$	231,417
Commercial paper	181,884		181,886
U.S. government or government agency securities	265,251		265,261
	$	678,552	$	678,564

	As of December 31, 2010
	Cost		Fair Value
Money market funds	$	168,831	$	168,831
Commercial paper	17,494		17,494
U.S. government or government agency securities	108,364		108,364
	$	294,689	$	294,689