Cash and Cash Equivalents and Investments (Tables)

3 Months Ended

Mar. 31, 2018

Cash and Cash Equivalents and Investments [Abstract]

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

	As of March 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
	(in thousands)
Cash and cash equivalents	$	20,421	$	—	$	—		$	20,421
Securities maturing within one year:
U.S. treasury securities	214,302		—		(317		)	213,985
Corporate debt securities	27,943		—		(77		)	27,866
Total short-term investments	$	242,245	$	—	$	(394	)	$	241,851
Total cash and cash equivalents and investments	$	262,666	$	—	$	(394	)	$	262,272
	As of December 31, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
			(in thousands)
Cash and cash equivalents	$	61,661	$	—	$	—		$	61,661
Securities maturing within one year:
U.S. treasury securities	222,316		—		(168		)	222,148
Corporate debt securities	27,033		—		(54		)	26,979
Total short-term investments	$	249,349	$	—	$	(222	)	$	249,127
Total cash and cash equivalents and investments	$	311,010	$	—	$	(222	)	$	310,788