Cash and Cash Equivalents and Investments (Tables)

6 Months Ended

Jun. 30, 2015

Cash and Cash Equivalents and Investments [Abstract]

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

	As of June 30, 2015
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
	(in thousands)
Cash and cash equivalents	$	9,744	$	—	$	—		$	9,744
Securities maturing within one year:
Certificates of deposit	553		—		—			553
U.S. treasury securities	272,088		1,241		(1,143		)	272,186
Total short-term investments	$	272,641	$	1,241	$	(1,143	)	$	272,739
Total cash and cash equivalents and investments	$	282,385	$	1,241	$	(1,143	)	$	282,483
	As of December 31, 2014
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
			(in thousands)
Cash and cash equivalents	$	137,266	$	—	$	—		$	137,266
Securities maturing within one year:
Certificates of deposit	552		—		—			552
U.S. treasury securities	201,584		3		(66		)	201,521
Total short-term investments	$	202,136	$	3	$	(66	)	$	202,073
Total cash and cash equivalents and investments	$	339,402	$	3	$	(66	)	$	339,339