Cash and Cash Equivalents and Investments (Tables)

9 Months Ended

Sep. 30, 2018

Cash and Cash Equivalents and Investments [Abstract]

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

	As of September 30, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
	(in thousands)
Cash and cash equivalents	$	48,902	$	—	$	—		$	48,902
Securities maturing within one year:
U.S. treasury securities	125,651		—		(63		)	125,588
Corporate debt securities	12,815		—		(8		)	12,807
Total short-term investments	$	138,466	$	—	$	(71	)	$	138,395
Total cash and cash equivalents and investments	$	187,368	$	—	$	(71	)	$	187,297
	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