Marketable Securities

Marketable securities as of June 30, 2018 and December 31, 2017 consist of both available-for-sale debt securities of investment grade issuers (see below and Note 6) as well as equity securities of publicly traded companies (see Note 6).

The following tables summarize the Company's investments in available-for-sale debt securities:



	Amortized		Unrealized					Fair
As of June 30, 2018	Cost Basis		Gains		Losses			Value
Available-for-sale debt securities:
Corporate bonds	$	2,497,276	$	2,282	$	(18,987	)	$	2,480,571
U.S. government and government agency obligations	126,395		30		(1,589		)	124,836
Municipal bonds	2,572		—		(8		)	2,564
Commercial paper	76,013		—		—			76,013
Certificates of deposit	40,773		—		—			40,773
	$	2,743,029	$	2,312	$	(20,584	)	$	2,724,757

As of December 31, 2017
Available-for-sale debt securities:
Corporate bonds	$	1,717,976	$	2,176	$	(7,672	)	$	1,712,480
U.S. government and government agency obligations	186,699		34		(1,241		)	185,492
Municipal bonds	4,600		—		(13		)	4,587
Commercial paper	106,973		—		—			106,973
Certificates of deposit	11,024		—		—			11,024
	$	2,027,272	$	2,210	$	(8,926	)	$	2,020,556

The Company classifies its investments in available-for-sale debt securities based on their contractual maturity dates. The available-for-sale debt securities listed as of June 30, 2018 mature at various dates through March 2023. The fair values of available-for-sale debt security investments by contractual maturity consist of the following:



	June 30, 2018		December 31, 2017
Maturities within one year	$	765,642	$	593,783
Maturities after one year through five years	1,959,115		1,426,773
	$	2,724,757	$	2,020,556

The following table shows the fair value of the Company's available-for-sale debt securities that have unrealized losses and that are deemed to be only temporarily impaired, aggregated by investment category and length of time that the individual securities have been in a continuous unrealized loss position.



	Less than 12 Months					12 Months or Greater					Total
As of June 30, 2018	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss			Fair Value		Unrealized Loss
Corporate bonds	$	1,588,876	$	(15,052	)	$	218,109	$	(3,935	)	$	1,806,985	$	(18,987	)
U.S. government and government agency obligations	45,936		(656		)	71,864		(933		)	117,800		(1,589		)
Municipal bonds	2,061		(8		)	—		—			2,061		(8		)
	$	1,636,873	$	(15,716	)	$	289,973	$	(4,868	)	$	1,926,846	$	(20,584	)

As of December 31, 2017
Corporate bonds	$	930,970	$	(4,924	)	$	256,750	$	(2,748	)	$	1,187,720	$	(7,672	)
U.S. government and government agency obligations	110,532		(409		)	67,921		(832		)	178,453		(1,241		)
Municipal bonds	2,582		(10		)	2,005		(3		)	4,587		(13		)
	$	1,044,084	$	(5,343	)	$	326,676	$	(3,583	)	$	1,370,760	$	(8,926	)

There were no realized losses on sales of marketable securities, and realized gains were not material, for the three and six months ended June 30, 2018 and 2017.

With respect to marketable securities, for the three and six months ended June 30, 2018 and 2017, amounts reclassified from Accumulated other comprehensive (loss) income into Other income, net were related to realized gains on sales. The Company adopted ASU 2016-01 (see Note 1) during the first quarter of 2018; as a result, there were $16.5 million and $25.9 million of net unrealized gains on equity securities recognized in Other income, net for the three and six months ended June 30, 2018, respectively. For the three and six months ended June 30, 2017, there were $5.8 million and $11.2 million, respectively, of net unrealized gains on equity securities that were recorded in Other comprehensive income (loss).