Marketable Securities - (Tables)

12 Months Ended

Dec. 31, 2017

Marketable Securities [Abstract]

Schedule of Marketable Securities by Category

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

	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses		Fair Value
December 31, 2017
Municipal notes and bonds	$	109,750	$	—	$	161	$	109,589
Total marketable securities	$	109,750	$	—	$	161	$	109,589
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 Amortized Cost and Estimated Fair Value of Marketable Securities Classified by Maturity Date

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, 2017 and 2016:

	December 31, 2017				December 31, 2016
	Amortized Cost		Fair Value		Amortized Cost		Fair Value
Due within one year	$	104,742	$	104,605	$	82,499	$	82,445
Due after one through five years		5,008		4,984		5,431		5,427
Due after five through ten years		—		—		—		—
Due after ten years		—		—		—		—
Total marketable securities	$	109,750	$	109,589	$	87,930	$	87,872

Summary of Estimated Fair Value and Gross Unrealized Holding Losses of Marketable Securities in Unrealized Loss Position

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, 2017 and 2016.

	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, 2017
Municipal notes and bonds	$	98,805	$	161	$	—	$	—
Total marketable securities	$	98,805	$	161	$	—	$	—
December 31, 2016
Municipal notes and bonds	$	64,918	$	65	$	—	$	—
Total marketable securities	$	64,918	$	65	$	—	$	—