Fair Value Measurements

3 Months Ended

Mar. 31, 2012

Fair Value Measurements [Abstract]

Fair Value Measurements

3. Fair Value Measurements

Our financial instruments consist of cash equivalents, short-term and long-term marketable securities and accounts receivable. Accounts receivable, net of bad debt allowance, are stated at their carrying value, which approximates fair value due to the short time to expected receipt of cash.

Cash equivalents and short-term and long-term marketable securities, consisting of money market funds and U.S. government and corporate debt securities, are carried at fair value, which is defined as an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between knowledgeable and willing market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that knowledgeable and willing market participants would use in pricing an asset or liability. We use a three-tier value hierarchy, which prioritizes based on the inputs used in measuring fair value as follows:

Level 1—Observable inputs that reflect quoted prices (unadjusted) for identical assets or liabilities in active markets.

Level 2—Includes other inputs that are directly or indirectly observable in the marketplace.

Level 3—Unobservable inputs that are supported by little or no market activity.

The composition of our securities among the three levels of the fair value hierarchy is as follows (in thousands):

	March 31, 2012
	Level 1		Level 2		Level 3		Total
	(unaudited)
Assets:
Money market funds	$	214,913	$	—	$	—	$	214,913
U.S. government debt securities		—		779,447		—		779,447
Corporate debt securities		—		403,447		—		403,447
Total	$	214,913	$	1,182,894	$	—	$	1,397,807
	December 31, 2011
	Level 1		Level 2		Level 3		Total
Assets:
Money market funds	$	1,375,918	$	—	$	—	$	1,375,918
U.S. government debt securities		—		267,635		—		267,635
Corporate debt securities		—		68,334		—		68,334
Total	$	1,375,918	$	335,969	$	—	$	1,711,887