Fair Value Measurements Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Marketable Securities [Table Text Block]

Fair Value of Instruments Measured and Recognized at Fair Value

The following tables summarize the levels of fair value measurements of our financial assets:

	December 31, 2018						December 31, 2017
(in thousands)	Total		Level 1		Level 2		Total		Level 1		Level 2
Money market funds	$	327,869	$	327,869	$	—	$	360,746	$	360,746	$	—
U.S. Treasury bills	50,147		—		50,147		—		—		—
Municipal bonds	10,634		—		10,634		1,960		—		1,960
Total	$	388,650	$	327,869	$	60,781	$	362,706	$	360,746	$	1,960

Available-for-sale Securities [Table Text Block]

The following is a summary of our available-for-sale marketable securities:

(in thousands)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
December 31, 2018
U.S. Treasury bills	$	50,150	$	—	$	(3	)	$	50,147
Municipal bonds	10,641		—		(7		)	10,634
December 31, 2017
U.S. Treasury bills	$	—	$	—	$	—		$	—
Municipal bonds	1,965		—		(5		)	1,960