Cash and Cash Equivalents and Investments (Tables)

6 Months Ended

Jun. 30, 2018

Cash and Cash Equivalents and Investments [Abstract]

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

	As of June 30, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
	(in thousands)
Cash and cash equivalents	$	15,044	$	—	$	—		$	15,044
Securities maturing within one year:
U.S. treasury securities	174,262		—		(180		)	174,082
Corporate debt securities	20,601		—		(38		)	20,563
Total short-term investments	$	194,863	$	—	$	(218	)	$	194,645
Total cash and cash equivalents and investments	$	209,907	$	—	$	(218	)	$	209,689
	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