Cash and Cash Equivalents and Investments (Tables)

6 Months Ended

Jun. 30, 2017

Cash and Cash Equivalents and Investments [Abstract]

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

	As of June 30, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
	(in thousands)
Cash and cash equivalents	$	30,706	$	—	$	—		$	30,706
Securities maturing within one year:
U.S. treasury securities	161,020		—		(178		)	160,842
Corporate debt securities	39,675		—		(36		)	39,639
Total short-term investments	$	200,695	$	—	$	(214	)	$	200,481
Total cash and cash equivalents and investments	$	231,401	$	—	$	(214	)	$	231,187
	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