Fair Value Measurements (Tables)

6 Months Ended

Jun. 30, 2012

Fair Value Measurements

Estimated fair values of the financial assets

	Fair Value Measurements at Reporting Date Using
June 30, 2012	Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs		Significant Unobservable Inputs
(in thousands)	Level 1		Level 2		Level 3		Total
U.S. government securities	$	30,255	$	—	$	—	$	30,255
U.S. government agency securities	75,632		36,564		—		112,196
U.S. corporate notes	57,066		3,086		—		60,152
U.S. commercial paper	—		44,993		—		44,993
Money market funds	129,891		—		—		129,891
Total	$	292,844	$	84,643	$	—	$	377,487

	Fair Value Measurements at Reporting Date Using
December 31, 2011	Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs		Significant Unobservable Inputs
(in thousands)	Level 1		Level 2		Level 3		Total
U.S. government securities	$	66,174	$	—	$	—	$	66,174
U.S. government agency securities	55,901		37,274		—		93,175
U.S. corporate notes	2,705		—		—		2,705
U.S. commercial paper	—		34,976		—		34,976
Money market funds	38,721		—		—		38,721
Total	$	163,501	$	72,250	$	—	$	235,751