MARKETABLE SECURITIES

As of December 31, 2018 and 2017, our marketable securities consisted of securities classified as available-for-sale in accordance with accounting standards which provide guidance related to accounting and classification of certain investments in marketable securities.

The following is a summary of our marketable securities as of December 31, 2018 and 2017 (in thousands):



	December 31, 2018
			Gross		Gross			Estimated
	Amortized		Unrealized		Unrealized			Fair
Description of Securities:	Cost		Gains		Losses			Value
Short-term investments:*
Corporate debt securities	$	51,184	$	—	$	(236	)	$	50,948
U.S. treasury and government agency securities	7,647		—		(34		)	7,613
Commercial paper	3,995		—		—			3,995
Certificates of deposit	12,000		—		—			12,000
Total short-term investments	74,826		—		(270		)	74,556
Long-term investments:**
Corporate debt securities	62,530		52		(433		)	62,149
U.S. treasury and government agency securities	2,742		—		(32		)	2,710
Certificates of deposit	1,500		—		—			1,500
Total long-term investments	66,772		52		(465		)	66,359
Total investments	$	141,598	$	52	$	(735	)	$	140,915



	December 31, 2017
			Gross		Gross			Estimated
	Amortized		Unrealized		Unrealized			Fair
Description of Securities:	Cost		Gains		Losses			Value
Short-term investments:*
Corporate debt securities	$	57,257	$	—	$	(68	)	$	57,189
U.S. treasury and government agency securities	1,999		—		(13		)	1,986
Commercial paper	1,999		—		—			1,999
Certificates of deposit	9,151		—		—			9,151
Total short-term investments	70,406		—		(81		)	70,325
Long-term investments:**
Corporate debt securities	59,282		1		(320		)	58,963
U.S. treasury and government agency securities	7,381		—		(76		)	7,305
Total long-term investments	66,663		1		(396		)	66,268
Total investments	$	137,069	$	1	$	(477	)	$	136,593

* Represents marketable securities with a remaining maturity of less than one year.

** Represents marketable securities with a remaining maturity of one to three years classified as short-term on our consolidated balance sheets.

Impairments and Unrealized Gains and Losses on Marketable Securities

We did not recognize any other-than-temporary impairment losses in our consolidated statements of operations related to our marketable securities during 2018, 2017 and 2016. We considered various factors, including the length of time that each security was in a realized loss position and our ability and intent to hold these securities until recovery of their amortized cost basis occurs. As of December 31, 2018, we have no material losses in an unrealized loss position for more than one year. Future events may occur, or additional information may become available, which may cause us to identify credit losses where we do not expect to receive cash flows sufficient to recover the entire amortized cost basis of a security and may necessitate the recording of future realized losses on securities in our portfolio. Significant losses in the estimated fair values of our marketable securities could have a material adverse effect on our earnings in future periods.