Marketable Securities

Marketable securities as of March 31, 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 March 31, 2018	Cost Basis		Gains		Losses			Value
Available-for-sale debt securities:
Corporate bonds	$	2,126,820	$	1,948	$	(18,159	)	$	2,110,609
U.S. government and government agency obligations	156,281		30		(1,611		)	154,700
Municipal bonds	2,591		—		(11		)	2,580
Commercial paper	70,385		—		—			70,385
Certificates of deposit	20,097		—		—			20,097
	$	2,376,174	$	1,978	$	(19,781	)	$	2,358,371

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 debt securities based on their contractual maturity dates. The debt securities listed as of March 31, 2018 mature at various dates through March 2023. The fair values of debt security investments by contractual maturity consist of the following:



	March 31, 2018		December 31, 2017
Maturities within one year	$	605,461	$	593,783
Maturities after one year through five years	1,752,910		1,426,773
	$	2,358,371	$	2,020,556

The following table shows the fair value of the Company's 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 March 31, 2018	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss			Fair Value		Unrealized Loss
Corporate bonds	$	1,343,778	$	(13,950	)	$	234,258	$	(4,209	)	$	1,578,036	$	(18,159	)
U.S. government and government agency obligations	70,872		(623		)	76,793		(988		)	147,665		(1,611		)
Municipal bonds	2,579		(11		)	—		—			2,579		(11		)
	$	1,417,229	$	(14,584	)	$	311,051	$	(5,197	)	$	1,728,280	$	(19,781	)

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 months ended March 31, 2018 and 2017.

With respect to marketable securities, for the three months ended March 31, 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 was $9.4 million of unrealized gains on equity securities recognized during the three months ended March 31, 2018 that was recorded in Other income, net. For the three months ended March 31, 2017, there was $5.4 million of unrealized gains and losses on equity securities that was recorded in Other comprehensive income (loss).