Fair Value Measurements

12 Months Ended

Dec. 31, 2011

Fair Value Measurements

Fair Value Measurements

5. Fair Value Measurements

        The Company defines fair value as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date.

        The Company's valuation techniques are based on observable and unobservable inputs. Observable inputs reflect readily obtainable data from independent sources, while unobservable inputs reflect the Company's market assumptions. The Company classifies these inputs into the following hierarchy:

•         Level 1 Inputs— Quoted prices for identical instruments in active markets.

          Level 2 Inputs— Quoted prices for similar instruments in active markets; quoted prices for identical or similar instruments in markets that are not active; and model-derived valuations whose inputs are observable or whose significant value drivers are observable.

          Level 3 Inputs— Unobservable inputs and little, if any, market activity for the assets.

        The estimated fair values of the Company's financial assets were as follows:

	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