Our marketable investment securities portfolio consists of various debt and equity instruments summarized in the table below. Certain of our investments in debt and equity instruments have historically experienced and are likely to continue experiencing volatility.



	As of
	March 31, 2019		December 31, 2018
	(In thousands)
Marketable investment securities:
Debt securities:
Corporate bonds	$	1,470,693	$	1,735,653
Other debt securities	400,125		464,997
Total debt securities	1,870,818		2,200,650
Equity securities	64,887		90,976
Total marketable investment securities	1,935,705		2,291,626
Less: Restricted marketable investment securities	10,597		9,474
Total marketable investment securities	$	1,925,108	$	2,282,152

Our corporate bond portfolio includes debt instruments issued by individual corporations, primarily in the industrial and financial services industries. Our other debt securities portfolio includes investments in various debt instruments, including U.S. government bonds, commercial paper and mutual funds.

A summary of our available-for-sale debt securities, exclusive of securities where we have elected the fair value option, is presented in the table below.



	Amortized		Unrealized					Estimated
	Cost		Gains		Losses			Fair Value
	(In thousands)
As of March 31, 2019
Corporate bonds	$	1,416,632	$	1,287	$	(120	)	$	1,417,799
Other debt securities	400,129		13		(17		)	400,125
Total available-for-sale debt securities	$	1,816,761	$	1,300	$	(137	)	$	1,817,924
As of December 31, 2018
Corporate bonds	$	1,689,093	$	318	$	(1,896	)	$	1,687,515
Other debt securities	464,993		7		(3		)	464,997
Total available-for-sale debt securities	$	2,154,086	$	325	$	(1,899	)	$	2,152,512

As of March 31, 2019, corporate bonds where we have elected the fair value option have a fair value of $53 million. We recognized gains of $4 million on these securities for the three months ended March 31, 2019, respectively. We had no debt securities that were accounted for using the fair value option during the three months ended March 31, 2018.

As of March 31, 2019, we have $1.6 billion of available-for-sale debt securities with contractual maturities of one year or less and $186 million with contractual maturities greater than one year.

Our marketable equity securities consist primarily of shares of common stock of public companies. For the three months ended March 31, 2019 and 2018, Gains (losses) on investments, net included net gains of $30 million and net losses of $37 million, respectively, related to equity securities that we held each period. The fair value of our equity securities was $65 million and $91 million as of March 31, 2019 and December 31, 2018, respectively.

Proceeds from sales of our available-for-sale securities, including securities accounted for using the fair value option, were $436 million and zero for the three months ended March 31, 2019 and 2018, respectively. Sales of securities accounted for using the fair value option do not result in gains or losses because we recognize unrealized gains and losses on such securities prior to the time of sale.

Our marketable investment securities are measured at fair value on a recurring basis as summarized in the table below. As of March 31, 2019 and December 31, 2018, we did not have investments that were categorized within Level 3 of the fair value hierarchy.



	As of
	March 31, 2019						December 31, 2018
	Level 1		Level 2		Total		Level 1		Level 2		Total
	(In thousands)
Debt securities:
Corporate bonds	$	—	$	1,470,693	$	1,470,693	$	—	$	1,735,653	$	1,735,653
Other	10,597		389,528		400,125		9,474		455,523		464,997
Total debt securities	10,597		1,860,221		1,870,818		9,474		2,191,176		2,200,650
Equity securities	58,400		6,487		64,887		85,298		5,678		90,976
Total marketable investment securities	$	68,997	$	1,866,708	$	1,935,705	$	94,772	$	2,196,854	$	2,291,626