Cash and Cash Equivalents and Investments (Tables)

3 Months Ended

Mar. 31, 2016

Cash and Cash Equivalents and Investments [Abstract]

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

	As of March 31, 2016
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
	(in thousands)
Cash and cash equivalents	$	24,681	$	—	$	—		$	24,681
Securities maturing within one year:
U.S. treasury securities	434,505		305		(7		)	434,803
Corporate debt securities	17,663		2		(3		)	17,662
Total short-term investments	$	452,168	$	307	$	(10	)	$	452,465
Total cash and cash equivalents and investments	$	476,849	$	307	$	(10	)	$	477,146
	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