Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 2012

Fair Value Measurements

Estimated fair values of the financial assets

	Fair Value Measurements at Reporting Date Using
March 31, 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	$	25,998	$	35,033	$	—	$	61,031
U.S. government agency securities	34,441		48,488		—		82,929
U.S. corporate notes	—		550		—		550
U.S. commercial paper	—		45,737		—		45,737
Money market funds	6,213		—		—		6,213
Total	$	66,652	$	129,808	$	—	$	196,460

	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