The following tables summarize the available-for-sale debt securities held at September 30, 2023 and December 31, 2022 (in thousands):


Description	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
September 30, 2023
US treasury securities	$	26,415	$	—	$	(298	)	$	26,117
US government agencies		192,787		1		(1,257	)		191,531
Corporate securities		137,009		—		(895	)		136,114
Total	$	356,211	$	1	$	(2,450	)	$	353,762


Description	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
December 31, 2022
US treasury securities	$	94,958	$	3	$	(1,111	)	$	93,850
US government agencies		180,967		25		(1,873	)		179,119
Corporate securities		220,119		25		(2,014	)		218,130
Total	$	496,044	$	53	$	(4,998	)	$	491,099

As of September 30, 2023, the Company held 106 securities that had been in an unrealized loss position for less than 12 months with an aggregate fair value of $231.5 million. As of December 31, 2022, the Company held 149 securities that had been in an unrealized loss position for less than 12 months with an aggregate fair value of $330.9 million. As of September 30, 2023, the Company held 56 securities that had been in an unrealized loss position for greater than 12 months with an aggregate fair value of $114.8 million. As of December 31, 2022, the Company held 36 securities that had been in an unrealized loss position for greater than 12 months with an aggregate fair value of $115.0 million.

As of September 30, 2023 the Company had 125 securities with a fair value of $276.8 million with a contractual maturity of less than 12 months and 39 securities with a fair value of $77.0 million with a contractual maturity of greater than 12 months. As of December 31, 2022 the Company had 118 securities with a fair value of $338.8 million with a contractual maturity of less than 12 months and 81 securities with a fair value of $152.3 million with a contractual maturity of greater than 12 months. There were no sales of marketable securities during the nine months ending September 30, 2023 and 2022.

The Company is required to determine whether a decline in the fair value below the amortized cost basis of available-for-sale securities is due to credit-related factors. At each reporting date, the Company performs an evaluation of impairment to determine if any unrealized losses are the result of credit losses. Impairment is assessed at the individual security level. Factors considered in determining whether a loss resulted from a credit loss or other factors include the Company’s intent and ability to hold the investment until the recovery of its amortized cost basis, the extent to which the fair value is less than the amortized cost basis, the length of time and extent to which fair value has been less than the cost basis, the financial condition of the issuer, any historical failure of the issuer to make scheduled interest or principal payments, any changes to the rating of the security by a rating agency, any adverse legal or regulatory events affecting the issuer or issuer’s industry, and any significant deterioration in economic conditions.

Unrealized losses on available-for-sale securities presented in the previous table have not been recognized in the condensed consolidated statements of operations because the securities are high credit quality, investment grade securities that the Company does not intend to sell and will not be required to sell prior to their anticipated recovery, and the decline in fair value is attributable to factors other than credit losses. Based on its evaluation, the Company determined it does not have any credit losses related to its available-for-sale securities as of September 30, 2023 and December 31, 2022.