Fair Value Measurements Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2017

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:

	Fair Value Measurements
			(in thousands)
	December 31, 2017		Level 1		Level 2		Level 3
Money market funds	$	360,746	$	360,746	$	—	$	—
Municipal bonds	1,960		—		1,960		—
Total	$	362,706	$	360,746	$	1,960	$	—

	Fair Value Measurements
			(in thousands)
	December 31, 2016		Level 1		Level 2		Level 3
Money market funds	$	283,322	$	283,322	$	—	$	—
Municipal bonds	1,851		—		1,851		—
Total	$	285,173	$	283,322	$	1,851	$	—

Available-for-sale Securities [Table Text Block]

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

	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
			(in thousands)
December 31, 2017
Municipal bonds	$	1,965	$	—	$	(5	)	$	1,960
December 31, 2016
Municipal bonds	$	1,854	$	—	$	(3	)	$	1,851

Investments Classified by Contractual Maturity Date [Table Text Block]

As of December 31, 2017, the contractual maturities of our marketable securities were as follows:

	Amortized Cost		Estimated Fair Value
	(in thousands)
Less than one year	$	1,860	$	1,856
One to five years	105		104
Total	$	1,965	$	1,960