Cash and Cash Equivalents and Investments (Tables)

3 Months Ended

Mar. 31, 2017

Cash and Cash Equivalents and Investments [Abstract]

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

	As of March 31, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
	(in thousands)
Cash and cash equivalents	$	2,455	$	—	$	—		$	2,455
Securities maturing within one year:
U.S. treasury securities	191,443		—		(195		)	191,248
Corporate debt securities	66,109		—		(69		)	66,040
Total short-term investments	$	257,552	$	—	$	(264	)	$	257,288
Total cash and cash equivalents and investments	$	260,007	$	—	$	(264	)	$	259,743
	As of December 31, 2016
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
			(in thousands)
Cash and cash equivalents	$	46,600	$	—	$	—		$	46,600
Securities maturing within one year:
U.S. treasury securities	227,911		1		(107		)	227,805
Corporate debt securities	72,188		1		(90		)	72,099
Total short-term investments	$	300,099	$	2	$	(197	)	$	299,904
Total cash and cash equivalents and investments	$	346,699	$	2	$	(197	)	$	346,504