Marketable Securities

9 Months Ended

Sep. 30, 2017

Investments Debt And Equity Securities [Abstract]

Marketable securities

3. Marketable securities

The following table summarizes the available-for-sale securities held at September 30, 2017 and December 31, 2016 (in thousands):

Description	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
September 30, 2017
U.S. government agency securities and treasuries	$	887,586	$	5	$	(1,405	)	$	886,186
Certificates of deposit		18,480		2		—			18,482
Total	$	906,066	$	7	$	(1,405	)	$	904,668
December 31, 2016
U.S. government agency securities and treasuries	$	600,001	$	34	$	(575	)	$	599,460
Certificates of deposit		6,480		6		(3	)		6,483
Total	$	606,481	$	40	$	(578	)	$	605,943

No available-for-sale securities held as of September 30, 2017 or December 31, 2016 had remaining maturities greater than three years.