Marketable Securities (Tables)

12 Months Ended

Dec. 31, 2016

Marketable Securities [Abstract]

Available-for-sale Securities [Table Text Block]

At December 31, 2016 and 2015, marketable securities are categorized as follows:

	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Fair Value
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
December 31, 2015
Municipal notes and bonds	$	116,086	$	18	$	(15	)	$	116,089
Corporate bonds	838		—		(3		)	835
Total marketable securities	$	116,924	$	18	$	(18	)	$	116,924

Investments 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 Consolidated Balance Sheet classification, is as follows at December 31, 2016 and 2015:

	December 31, 2016				December 31, 2015
	Amortized Cost		Fair Value		Amortized Cost		Fair Value
Due within one year	$	82,498	$	82,445	$	113,542	$	113,549
Due after one through five years	5,431		5,427		3,382		3,375
Due after five through ten years	—		—		—		—
Due after ten years	—		—		—		—
Total marketable securities	$	87,929	$	87,872	$	116,924	$	116,924

Schedule of Unrealized Loss on Investments [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 December 31, 2016 and 2015.

	In Unrealized Loss Position For Less Than 12 Months					In Unrealized Loss Position For Greater Than 12 Months
	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
December 31, 2016
Municipal notes and bonds	$	64,918	$	(65	)	$	—	$	—
Corporate bonds	—		—			—		—
Total marketable securities	$	64,918	$	(65	)	$	—	$	—
December 31, 2015
Municipal notes and bonds	$	52,638	$	(15	)	$	305	$	(1	)
Corporate bonds	835		(3		)	—		—
Total marketable securities	$	53,473	$	(18	)	$	305	$	(1	)