Marketable Securities

9 Months Ended

Sep. 30, 2015

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, 2015
			Gross		Gross
	Amortized		Unrealized		Unrealized			Aggregate
	Cost		Gains		Losses			Fair Value
U.S. government and government agency debt securities	$	196,828	$	86	$	(1	)	$	196,913
Corporate debt securities		214,276		50		(23	)		214,303
Total	$	411,104	$	136	$	(24	)	$	411,216
	December 31, 2014
			Gross		Gross
	Amortized		Unrealized		Unrealized			Aggregate
	Cost		Gains		Losses			Fair Value
U.S. government and government agency debt securities	$	405,049	$	68	$	(135	)	$	404,982
Corporate debt securities		611,950		39		(365	)		611,624
Total	$	1,016,999	$	107	$	(500	)	$	1,016,606

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, 2015
Due within one year	$	406,701
After one year through three years		4,515
Total	$	411,216

Changes in market interest rates and bond yields caused certain investments to fall below their cost basis, resulting in unrealized losses on marketable securities. As of September 30, 2015, we had unrealized losses of $0.1 million related to marketable securities that had a fair value of $100.1 million. As of December 31, 2014, we had unrealized losses of $0.5 million related to marketable securities that had a fair value of $621.5 million. None of these securities were in a material continuous unrealized loss position for more than 12 months.

As of September 30, 2015 and December 31, 2014, 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 to maturity and whether it is more likely than not that we will be required to sell the investment before recovery of its cost basis.