Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2011

Fair Value Measurements

Estimated fair values of the financial assets

	Fair Value Measurements at Reporting Date Using
	Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs		Significant Unobservable Inputs
December 31, 2011 (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

	Fair Value Measurements at Reporting Date Using
	Quoted Prices in Active Markets for Identical Assets		Significant Other Observable Inputs		Significant Unobservable Inputs
December 31, 2010 (in thousands)	Level 1		Level 2		Level 3		Total
U.S. government securities	$	25,976	$	—	$	—	$	25,976
U.S. government agency securities		24,375		30,273		—		54,648
U.S. corporate notes		34,695		—		—		34,695
U.S. commercial paper		—		97,221		—		97,221
Money market funds		91,805		—		—		91,805
Total	$	176,851	$	127,494	$	—	$	304,345