Marketable Securities (Tables)

12 Months Ended

Dec. 31, 2015

Marketable Securities [Abstract]

Available-for-sale Securities [Table Text Block]

At December 31, 2015 and 2014, marketable securities are categorized as follows:

	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Fair Value
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
December 31, 2014
Municipal notes and bonds	$	113,838	$	37	$	(4	)	$	113,871
Total marketable securities	$	113,838	$	37	$	(4	)	$	113,871

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, 2015 and 2014:

	December 31, 2015				December 31, 2014
	Amortized Cost		Fair Value		Amortized Cost		Fair Value
Due within one year	$	113,542	$	113,549	$	107,151	$	107,177
Due after one through five years	3,382		3,375		6,687		6,694
Due after five through ten years	—		—		—		—
Due after ten years	—		—		—		—
Total marketable securities	$	116,924	$	116,924	$	113,838	$	113,871

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, 2015 and 2014.

	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, 2015
Municipal notes and bonds	$	52,638	$	(15	)	$	305	$	(1	)
Corporate bonds	$	835	$	(3	)	$	—	$	—
Total marketable securities	$	53,473	$	(18	)	$	305	$	(1	)
December 31, 2014
Municipal notes and bonds	$	26,698	$	(4	)	$	—	$	—
Total marketable securities	$	26,698	$	(4	)	$	—	$	—