Financial Instruments (Tables)

6 Months Ended

Jun. 30, 2018

Investments Debt And Equity Securities [Abstract]

Cash Equivalents and Investments Classified as Available-for-sale Securities

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

	June 30, 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	$	27,208	$	—	$	—		$	27,208	$	31,836	$	—	$	—		$	31,836
Corporate notes and commercial paper		247,454		—		(373	)		247,081		313,307		2		(145	)		313,164
U.S. Treasury bills and government agency securities		163,422		3		(304	)		163,121		170,724		—		(266	)		170,458
	$	438,084	$	3	$	(677	)	$	437,410	$	515,867	$	2	$	(411	)	$	515,458
Classified as:
Cash equivalents								$	153,797								$	162,793
Short-term investments									262,836									281,589
Long-term investments									20,777									71,076
Total cash equivalents and investments								$	437,410								$	515,458