Marketable Securities

The following is a summary of available-for-sale securities as of March 31, 2019 (in thousands):



	Amortized Cost		Gross Unrealized Gain		Gross Unrealized (Losses)			Estimated Fair Value
Corporate debt securities	$	46,685	$	95	$	(89	)	$	46,691
Tax-exempt municipal securities	18,969		83		(9		)	19,043
Variable rate demand notes	1,600		—		—			1,600
Commercial paper	1,018		—		—			1,018
Total	$	68,272	$	178	$	(98	)	$	68,352

The following is a summary of available-for-sale securities as of December 31, 2018 (in thousands):



	Amortized Cost		Gross Unrealized Gain		Gross Unrealized (Losses)			Estimated Fair Value
Corporate debt securities	$	48,202	$	12	$	(284	)	$	47,930
Tax-exempt municipal securities	22,858		45		(34		)	22,869
Variable rate demand notes	4,000		—		—			4,000
Total	$	75,060	$	57	$	(318	)	$	74,799

The unrealized gains and losses on the Company’s investments in municipal and corporate debt securities as of December 31, 2018 and March 31, 2019 were caused by changes in market values primarily due to interest rate changes. As of March 31, 2019, the fair value of the Company’s securities which were in an unrealized loss position for a period longer than twelve months was $17.6 million. The Company does not intend to sell these securities, and it is not more likely than not that the Company will be required to sell these securities prior to the recovery of their amortized cost basis, which may be at maturity. No other-than-temporary impairment charges were recorded during the three months ended March 31, 2018 and 2019.

The following table summarizes the maturities of the Company’s marketable securities as of December 31, 2018 and March 31, 2019 (in thousands):



	December 31, 2018		March 31, 2019
Due within one year	$	37,121	$	36,486
Due after one year through five years	37,678		31,866
Total	$	74,799	$	68,352

Amounts due within one year in the table above included $1.6 million of variable rate demand notes, which have contractual maturities ranging from 26 years to 27 years as of March 31, 2019. The variable rate demand notes are floating rate municipal bonds with embedded put options that allow the Company to sell the security at par plus accrued interest on a settlement basis ranging from one day to seven days. The Company has classified these securities based on their effective maturity dates, which ranges from one day to seven days from the balance sheet date.

The Company received $12.9 million of proceeds from the maturities of available-for-sale securities during the three months ended March 31, 2019. The Company did not record any gross realized gains or gross realized losses in net income during the three months ended March 31, 2019 and 2018. Additionally, there were no proceeds from sales of marketable securities prior to maturity during the three months ended March 31, 2018 and 2019.