Fair Value Measurements Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 2019

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:

	March 31, 2019						December 31, 2018
(in thousands)	Total		Level 1		Level 2		Total		Level 1		Level 2
Money market funds	$	377,377	$	377,377	$	—	$	325,819	$	325,819	$	—
U.S. Treasury bills	62,272		62,272		—		52,197		52,197		—
Municipal bonds	4,567		—		4,567		10,634		—		10,634
Total	$	444,216	$	439,649	$	4,567	$	388,650	$	378,016	$	10,634

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
March 31, 2019
U.S. Treasury bills	$	49,028	$	8	$	(4	)	$	49,032
Municipal bonds	4,567		—		—			4,567
Total	$	53,595	$	8	$	(4	)	$	53,599
December 31, 2018
U.S. Treasury bills	$	50,150	$	—	$	(3	)	$	50,147
Municipal bonds	10,640		1		(7		)	10,634
Total	$	60,790	$	1	$	(10	)	$	60,781

Investments Classified by Contractual Maturity Date [Table Text Block]

As of March 31, 2019, the contractual maturities of our marketable securities were as follows:

(in thousands)	Amortized Cost		Estimated Fair Value
Less than one year	$	53,595	$	53,599
One to five years	—		—
Total	$	53,595	$	53,599