Fair Value Measurements (Tables)

6 Months Ended

Jun. 30, 2018

Fair Value Measurements [Abstract]

Fair Value, by Balance Sheet Grouping [Table Text Block]

	Assets and Liabilities at Fair Value as of June 30, 2018
	Level 1		Level 2		Level 3		Total
	(in thousands)
Assets
Cash and cash equivalents	$	15,044	$	—	$	—	$	15,044
Short-term investments	174,082		20,563		—		194,645
Total cash and cash equivalents and investments	$	189,126	$	20,563	$	—	$	209,689

	Assets and Liabilities at Fair Value as of December 31, 2017
	Level 1		Level 2		Level 3		Total
	(in thousands)
Assets
Cash and cash equivalents	$	61,661	$	—	$	—	$	61,661
Short-term investments	222,148		26,979		—		249,127
Total cash and cash equivalents and investments	$	283,809	$	26,979	$	—	$	310,788