Marketable Securities

3 Months Ended

Mar. 31, 2015

Investments Debt And Equity Securities [Abstract]

Marketable securities

3. Marketable securities

The following table summarizes the available-for-sale securities held at March 31, 2015 and December 31, 2014 (in thousands):

Description	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
March 31, 2015
U.S. government agency securities	$	323,863	$	19	$	(141	)	$	323,741
Certificates of deposit		12,400		3		(4	)		12,399
Total	$	336,263	$	22	$	(145	)	$	336,140
December 31, 2014
U.S. government agency securities	$	131,589	$	6	$	(59	)	$	131,536
Certificates of deposit		12,640		—		(18	)		12,622
Total	$	144,229	$	6	$	(77	)	$	144,158

No available-for-sale securities held as of March 31, 2015 or December 31, 2014 had remaining maturities greater than two years.