Financial Instruments (Tables)

9 Months Ended

Sep. 30, 2018

Investments Debt And Equity Securities [Abstract]

Summary of Available-for-Sale Securities

Cash equivalents and investments, all of which are classified as available-for-sale securities and restricted cash, consisted of the following (in thousands):

	September 30, 2018									December 31, 2017
	Cost		Unrealized Gain		Unrealized (Loss)			Estimated Fair Value		Cost			Unrealized Gain		Unrealized (Loss)			Estimated Fair Value
Money market funds	$	15,160	$	—	$	—		$	15,160		$	12,430	$	—	$	—		$	12,430
Corporate notes and      commercial paper		78,279		—		(38	)		78,241			85,553		—		(61	)		85,492
U.S. treasury securities		13,949		—		(13	)		13,936			34,505		—		(68	)		34,437
U.S. government agency      securities		33,423		—		(122	)		33,301			53,832		—		(141	)		53,691
	$	140,811	$	—	$	(173	)	$	140,638		$	186,320	$	—	$	(270	)	$	186,050
Classified as:
Cash equivalents								$	59,877									$	47,931
Short-term investments									80,321										115,318
Long-term investments									—										22,361
Restricted cash									440										440
Total cash equivalents,    restricted cash and    investments								$	140,638									$	186,050