Marketable Securities (Tables)

6 Months Ended

Jun. 30, 2018

Investments Debt And Equity Securities [Abstract]

Summary of Fair Value of Available-for-Sale Marketable Securities by Type of Security

As of June 30, 2018 and December 31, 2017 the fair value of available-for-sale marketable securities by type of security was as follows:

	June 30, 2018
(In thousands)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Commercial paper		12,938		—		—			12,938
U.S. government obligations	$	64,310	$	2	$	(89	)	$	64,223
Corporate bonds		88,897		—		(226	)		88,671
	$	166,145	$	2	$	(315	)	$	165,832

	December 31, 2017
(In thousands)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Commercial paper	$	22,436	$	—	$	—		$	22,436
U.S. government obligations		121,470	$	—		(136	)		121,334
Corporate bonds		152,630	$	—		(273	)		152,357
	$	296,536	$	—	$	(409	)	$	296,127