Marketable Securities

9 Months Ended

Sep. 30, 2013

Investments Debt And Equity Securities [Abstract]

Marketable Securities

2. Marketable Securities

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

	September 30, 2013
			Gross		Gross
	Amortized		Unrealized		Unrealized			Aggregate
	Cost		Gains		Losses			Fair Value
U.S. government and government agency debt securities	$	367,542	$	274	$	(9	)	$	367,807
Corporate debt securities		751,964		348		(214	)		752,098
Municipal securities		17,302		9		—			17,311
Total	$	1,136,808	$	631	$	(223	)	$	1,137,216
	December 31, 2012
			Gross		Gross
	Amortized		Unrealized		Unrealized			Aggregate
	Cost		Gains		Losses			Fair Value
U.S. government and government agency debt securities	$	464,517	$	303	$	(5	)	$	464,815
Corporate debt securities		795,962		524		(170	)		796,316
Municipal securities		5,234		—		(1	)		5,233
Total	$	1,265,713	$	827	$	(176	)	$	1,266,364

For more detail on our method for determining the fair value of our assets, see Note 3 – “Fair Value Measurements”

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

	September 30, 2013
Due within one year	$	711,502
After one year through three years		425,714
Total	$	1,137,216

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 September 30, 2013, we had unrealized losses of $0.2 million related to marketable securities that had a fair value of $287.9 million. As of December 31, 2012, we had unrealized losses of $0.2 million related to marketable securities that had a fair value of $417.7 million. None of these securities were in a continuous unrealized loss position for more than 12 months.

As of September 30, 2013 and December 31, 2012, 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, our ability and intent to hold the security and whether it is more likely than not that we will be required to sell the investment before recovery of its cost basis.