Marketable Securities

3 Months Ended

Mar. 31, 2012

Marketable Securities [Abstract]

Marketable Securities

2. Marketable Securities

The following tables summarize the Company's amortized cost, gross unrealized gains and losses and fair value of its available-for-sale investments in marketable securities (in thousands):

	March 31, 2012
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Aggregate Fair Value
	(unaudited)
U.S. government debt securities	$	779,654	$	70	$	(278	)	$	779,446
Corporate debt securities		380,677		529		(259	)		380,947
Total	$	1,160,331	$	599	$	(537	)	$	1,160,393
	December 31, 2011
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Aggregate Fair Value
U.S. government debt securities	$	267,635	$	53	$	(53	)	$	267,635
Corporate debt securities		67,657		35		(64	)		67,628
Total	$	335,292	$	88	$	(117	)	$	335,263

The estimated fair value of available-for-sale marketable securities, classified by their contractual maturities was as follows (in thousands):

	March 31, 2012
	(unaudited)
Due within one year	$	701,662
One year through three years		458,731
Total	$	1,160,393

Changes in market interest rates and bond yields cause certain of our investments to fall below their cost basis, resulting in unrealized losses on marketable securities. As of March 31, 2012, we had unrealized losses of $0.5 million related to marketable securities with a fair value of $806.7 million. As of December 31, 2011, we had unrealized losses of $0.1 million related to marketable securities with a fair value of $111.1 million. None of these securities were in a continuous unrealized loss position for more than 12 months.

As of March 31, 2012 and December 31, 2011, we did not consider any of our marketable securities to be other-than-temporarily impaired. When evaluating our investments for other-than-temporary impairment, we review factors such as the length of time and extent to which fair value has been below its cost basis, the financial condition of the issuer, the Company's ability and intent to hold the security and whether it is more likely than not that the Company will be required to sell the investment before recovery of its cost basis.