The following is a summary of marketable securities held at March 31, 2012 and December 31, 2011 (in thousands):



			Gross Unrealized							Classification on Balance Sheet
As of March 31, 2012	Cost		Gains		Losses			Aggregate Fair Value		Short-term Marketable Securities		Long-term Marketable Securities
Available-for-sale securities:
Certificates of deposit	$	100	$	—	$	—		$	100	$	56	$	44
Commercial paper	4,988		—		(9		)	4,979		4,979		—
Corporate debt securities	609,158		1,431		(562		)	610,027		249,778		360,249
U.S. government agency obligations	219,090		52		(252		)	218,890		5,005		213,885
	$	833,336	$	1,483	$	(823	)	$	833,996	$	259,818	$	574,178
			Gross Unrealized							Classification on Balance Sheet
As of December 31, 2011	Cost		Gains		Losses			Aggregate Fair Value		Short-term Marketable Securities		Long-term Marketable Securities
Available-for-sale securities:
Certificates of deposit	$	42	$	—	$	—		$	42	$	—	$	42
Corporate debt securities	524,515		873		(580		)	524,808		285,012		239,796
U.S. government agency obligations	145,995		78		(165		)	145,908		5,017		140,891
	$	670,552	$	951	$	(745	)	$	670,758	$	290,029	$	380,729

Tabular disclosure of available-for-sale securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt.

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- Definition

Tabular disclosure of the fair value measurement of assets and liabilities which includes [financial] instruments measured at fair value that are classified in stockholders' equity. Such assets and liabilities may be measured on a recurring or nonrecurring basis. The disclosures which may be required or desired include: (1) for assets and liabilities measured on a recurring basis, disclosure may include: (a) the fair value measurements at the reporting date; (b) the level within the fair value hierarchy in which the fair value measurements in their entirety fall, segregating fair value measurements using quoted prices in active markets for identical assets or liabilities (Level 1), significant other observable inputs (Level 2), and significant unobservable inputs (Level 3); (c) for fair value measurements using significant unobservable inputs (Level 3), a reconciliation of the beginning and ending balances, separately presenting changes during the period attributable to the following: (i) total gains or losses for the period (realized and unrealized), segregating those gains or losses included in earnings (or changes in net assets), and a description of where those gains or losses included in earnings (or changes in net assets) are reported in the statement of income (or activities); (ii) purchases, sales, issuances, and settlements (net); (iii) transfers in and transfers out of Level 3 (for example, transfers due to changes in the observability of significant inputs); (d) the amount of the total gains or losses for the period in subparagraph (c) (i) above included in earnings (or changes in net assets) that are attributable to the change in unrealized gains or losses relating to those assets and liabilities still held at the reporting date and a description of where those unrealized gains or losses are reported in the statement of income (or activities); (e) the valuation technique(s) used to measure fair value and a discussion of changes in valuation techniques, if any, during the period and (2) for assets and liabilities that are measured at fair value on a nonrecurring basis (for example, impaired assets) disclosure may include, in addition to (a) above: (a) the reasons for the fair value measurements recorded; (b) the same as (b) above; (c) for fair value measurements using significant unobservable inputs (Level 3), a description of the inputs and the information used to develop the inputs; and (d) the valuation technique(s) used to measure fair value and a discussion of changes, if any, in the valuation technique(s) used to measure similar assets and/or liabilities in prior periods. Where the quoted price in an active market for the identical liability is not available, the Level 1 input is the quoted price of an identical liability when traded as an asset.

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