Cash and Cash Equivalents and Investments (Tables)

6 Months Ended

Jun. 30, 2016

Cash and Cash Equivalents and Investments [Abstract]

Schedule of Cash, Cash Equivalents and Short-term Investments [Table Text Block]

	As of June 30, 2016
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
	(in thousands)
Cash and cash equivalents	$	38,410	$	—	$	—		$	38,410
Securities maturing within one year:
U.S. treasury securities	379,307		333		—			379,640
Corporate debt securities	11,312		6		(3		)	11,315
Total short-term investments	$	390,619	$	339	$	(3	)	$	390,955
Total cash and cash equivalents and investments	$	429,029	$	339	$	(3	)	$	429,365
	As of December 31, 2015
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
			(in thousands)
Cash and cash equivalents	$	202,989	$	—	$	—		$	202,989
Securities maturing within one year:
U.S. treasury securities	313,105		2		(219		)	312,888
Corporate debt securities	5,477		—		(2		)	5,475
Total short-term investments	$	318,582	$	2	$	(221	)	$	318,363
Total cash and cash equivalents and investments	$	521,571	$	2	$	(221	)	$	521,352