Marketable Securities (Tables)

12 Months Ended

Dec. 31, 2017

Investments, Debt and Equity Securities [Abstract]

Summary of marketable securities

The following is a summary of our marketable securities as of December 31, 2017 and 2016 (in thousands):

	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

	December 31, 2016
			Gross		Gross			Estimated
	Amortized		Unrealized		Unrealized			Fair
Description of Securities:	Cost		Gains		Losses			Value
Short-term investments:*
Corporate debt securities	$	106,430	$	3	$	(69	)	$	106,364
U.S. treasury and government agency securities	1,021		—		—			1,021
Commercial paper	40,560		—		—			40,560
Certificates of deposit	6,000		—		—			6,000
Total short-term investments	154,011		3		(69		)	153,945
Long-term investments:**
Corporate debt securities	139,742		32		(281		)	139,493
U.S. treasury and government agency securities	11,395		—		(52		)	11,343
Total long-term investments	151,137		32		(333		)	150,836
Total investments	$	305,148	$	35	$	(402	)	$	304,781

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

** Represents marketable securities with a remaining maturity of one to three years.