Financial Instruments (Tables)

12 Months Ended

Dec. 31, 2018

Investments Debt And Equity Securities [Abstract]

Cash Equivalents and Short-term and Long-term Investments Classified as Available-for-sale Securities

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

	December 31, 2018									December 31, 2017
								Estimated									Estimated
	Amortized		Unrealized		Unrealized			Fair		Amortized		Unrealized		Unrealized			Fair
	Cost		Gain		(Loss)			Value		Cost		Gain		(Loss)			Value
Money market funds	$	19,500	$	—	$	—		$	19,500	$	31,836	$	—	$	—		$	31,836
Corporate notes and commercial paper		166,363		1		(205	)		166,159		313,307		2		(145	)		313,164
U.S. Treasury bills and government agency securities		110,270		1		(81	)		110,190		170,724		—		(266	)		170,458
	$	296,133	$	2	$	(286	)	$	295,849	$	515,867	$	2	$	(411	)	$	515,458
Classified as:
Cash equivalents								$	117,836								$	162,793
Short-term investments									178,013									281,589
Long-term investments									—									71,076
Total cash equivalents and investments								$	295,849								$	515,458