Marketable Securities - (Tables)

6 Months Ended

Jun. 30, 2017

Schedule of Available-for-sale Securities [Line Items]

Schedule of marketable securities by category [Table Text Block]

At June 30, 2017 and December 31, 2016, marketable securities are categorized as follows:

	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Fair Value
June 30, 2017
Municipal notes and bonds	$	90,389	$	21	$	(7	)	$	90,403
Corporate bonds	841		—		—			841
Total marketable securities	$	91,230	$	21	$	(7	)	$	91,244
December 31, 2016
Municipal notes and bonds	$	87,088	$	6	$	(65	)	$	87,029
Corporate bonds	842		1		—			843
Total marketable securities	$	87,930	$	7	$	(65	)	$	87,872

Schedule of marketable securities classified by contractual maturity date [Table Text Block]

The amortized cost and estimated fair value of marketable securities classified by the maturity date listed on the security, regardless of the Condensed Consolidated Balance Sheet classification, is as follows at June 30, 2017 and December 31, 2016:

	June 30, 2017				December 31, 2016
	Amortized Cost		Fair Value		Amortized Cost		Fair Value
Due within one year	$	77,592	$	77,601	$	82,498	$	82,445
Due after one through five years	13,638		13,643		5,432		5,427
Due after five through ten years	—		—		—		—
Due after ten years	—		—		—		—
Total marketable securities	$	91,230	$	91,244	$	87,930	$	87,872

Schedule of marketable securities in unrealized loss positions [Table Text Block]

The following table summarizes the estimated fair value and gross unrealized holding losses of marketable securities, aggregated by investment instrument and period of time in an unrealized loss position at June 30, 2017 and December 31, 2016:  

	Unrealized Loss Position For Less Than 12 Months					Unrealized Loss Position For Greater Than 12 Months
	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
June 30, 2017
Municipal notes and bonds	$	26,062	$	(7	)	$	—	$	—
Corporate bonds	—		—			—		—
Total	$	26,062	$	(7	)	$	—	$	—
December 31, 2016
Municipal notes and bonds	$	64,918	$	(65	)	$	—	$	—
Corporate bonds	—		—			—		—
Total	$	64,918	$	(65	)	$	—	$	—