Financial Instruments (Tables)

6 Months Ended

Jun. 30, 2016

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, 2016									December 31, 2015
								Estimated									Estimated
			Unrealized		Unrealized			Fair				Unrealized		Unrealized			Fair
	Cost		Gain		(Loss)			Value		Cost		Gain		(Loss)			Value
Money market funds	$	32,579	$	—	$	—		$	32,579	$	22,074	$	—	$	—		$	22,074
Corporate notes and commercial paper		194,141		19		(2	)		194,158		242,089		3		(59	)		242,033
U.S. government agency securities		110,570		78		(2	)		110,646		180,970		1		(95	)		180,876
	$	337,290	$	97	$	(4	)	$	337,383	$	445,133	$	4	$	(154	)	$	444,983
Classified as:
Cash equivalents								$	123,168								$	171,310
Short-term investments									214,215									257,713
Long-term investments									—									15,960
Total cash equivalents and    investments								$	337,383								$	444,983