As of December 31, 2019, the fair value of available-for-sale marketable debt securities by type of security was as follows:

	December 31, 2019
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Marketable debt securities:
U.S. Treasury notes	$	42,795	$	73	$	—		$	42,868
U.S. government agency bonds		15,508		31		(1	)		15,538
	$	58,303	$	104	$	(1	)	$	58,406

The amortized cost and fair value of the Company’s available-for-sale securities by contractual maturity are summarized as follows:

	December 31, 2019
	Amortized Cost		Fair Value
Maturing in one year or less	$	58,303	$	58,406
	$	58,303	$	58,406

As of December 31, 2018, the fair value of available-for-sale marketable securities by type of security was as follows:

	December 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Marketable debt securities:
U.S. Treasury notes	$	62,866	$	—	$	(24	)	$	62,842
U.S. government agency bonds		2,900		—		(15	)		2,885
Corporate bonds		10,276		—		(80	)		10,196
	$	76,042	$	—	$	(119	)	$	75,923

The amortized cost and fair value of the Company’s available-for-sale securities by contractual maturity are summarized as follows:

	December 31, 2018
	Amortized Cost		Fair Value
Maturing in one year or less	$	76,042	$	75,923
	$	76,042	$	75,923

The cost of securities sold is determined based on the specific identification method for purposes of recording realized gains and losses. During the year ended December 31, 2019 realized gain on sales of marketable securities was $7. During the year ended December 31, 2018 there were no realized gains (losses) on sales of marketable securities. During the year ended December 31, 2017, realized losses on sales of marketable securities were $(2). There were no marketable securities that required adjustment for other-than-temporary declines in fair value during the years ended December 31, 2019, 2018, and 2017.

The aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months as of December 31, 2019 and 2018 was $8,031 and $62,842, respectively. There were no securities held in an unrealized loss position for more than twelve months as of December 31, 2019. The aggregate fair value of securities held by the Company in an unrealized loss position for more than twelve months as of December 31, 2018 was $13,081. The Company determined that there was no material change in the credit risk of these investments. As a result, the Company determined it did not hold any investments with an other-than-temporary decline in fair value as of December 31, 2019 and 2018.