Marketable Securities

3 Months Ended

Mar. 31, 2016

Investments Debt And Equity Securities [Abstract]

Marketable securities

3. Marketable securities

The following table summarizes the available-for-sale securities held at March 31, 2016 and December 31, 2015 (in thousands):

Description	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
March 31, 2016
U.S. government agency securities and treasuries	$	625,903	$	91	$	(370	)	$	625,624
Certificates of deposit		8,640		13		—			8,653
Total	$	634,543	$	104	$	(370	)	$	634,277
December 31, 2015
U.S. government agency securities and treasuries	$	689,425	$	22	$	(2,300	)	$	687,147
Certificates of deposit		14,360		—		(13	)		14,347
Total	$	703,785	$	22	$	(2,313	)	$	701,494

No available-for-sale securities held as of March 31, 2016 or December 31, 2015 had remaining maturities greater than three years.