Fair Value Measurements Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2018

Fair Value Disclosures [Abstract]

Fair Value, Assets Measured on Recurring and Nonrecurring Basis [Table Text Block]

The following table summarizes the levels of fair value measurements of our financial assets:

	September 30, 2018						December 31, 2017
(in thousands)	Total		Level 1		Level 2		Total		Level 1		Level 2
Money market funds	$	314,161	$	314,161	$	—	$	360,746	$	360,746	$	—
U.S. Treasury bills	20,867		—		20,867		—		—		—
Municipal bonds	16,709		—		16,709		1,960		—		1,960
Total	$	351,737	$	314,161	$	37,576	$	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
September 30, 2018
U.S. Treasury bills	$	20,869	$	—	$	(2	)	$	20,867
Municipal bonds	16,732		—		(23		)	16,709
Total	$	37,601	$	—	$	(25	)	$	37,576
December 31, 2017
U.S. Treasury bills	$	—	$	—	$	—		$	—
Municipal bonds	1,965		—		(5		)	1,960
Total	$	1,965	$	—	$	(5	)	$	1,960

Investments Classified by Contractual Maturity Date [Table Text Block]

As of September 30, 2018, the contractual maturities of our marketable securities were as follows:

(in thousands)	Amortized Cost		Estimated Fair Value
Less than one year	$	37,601	$	37,576
One to five years	—		—
Total	$	37,601	$	37,576