Cash and Cash Equivalents and Investments (Tables)

9 Months Ended

Sep. 30, 2017

Cash and Cash Equivalents and Investments [Abstract]

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

	As of September 30, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
	(in thousands)
Cash and cash equivalents	$	83,293	$	—	$	—		$	83,293
Securities maturing within one year:
U.S. treasury securities	90,533		—		(57		)	90,476
Corporate debt securities	23,054		—		(6		)	23,048
Total short-term investments	$	113,587	$	—	$	(63	)	$	113,524
Total cash and cash equivalents and investments	$	196,880	$	—	$	(63	)	$	196,817
	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