Fair Value of Financial Instruments (Tables)

12 Months Ended

Dec. 31, 2017

Fair Value Disclosures [Abstract]

Assets and liabilities measured at fair value

Assets and liabilities measured at fair value on a recurring basis included the following as of December 31, 2017 and 2016 (in thousands):

	December 31, 2017								December 31, 2016
	Fair Value		Level 1		Level 2		Level 3		Fair Value		Level 1		Level 2		Level 3
Cash equivalents	$	368,859	$	358,859	$	10,000	$	—	$	48,136	$	48,136	$	—	$	—
Corporate bonds	74,648		—		74,648		—		60,676		—		60,676		—
Agency bonds	—		—		—		—		28,930		—		28,930		—
U.S. treasury securities	188,880		—		188,880		—		157,686		—		157,686		—
Commercial paper	—		—		—		—		10,473		—		10,473		—
	$	632,387	$	358,859	$	273,528	$	—	$	305,901	$	48,136	$	257,765	$	—