The following tables summarize the available-for-sale debt securities held at March 31, 2024 and December 31, 2023 (in thousands):


Description	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
March 31, 2024
US treasury securities	$	187,847	$	—	$	(277	)	$	187,570
US government agencies		143,361		12		(423	)		142,950
Corporate securities		321,978		10		(1,084	)		320,904
Total	$	653,186	$	22	$	(1,784	)	$	651,424


Description	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
December 31, 2023
US treasury securities	$	23,361	$	5	$	(113	)	$	23,253
US government agencies		142,948		48		(305	)		142,691
Corporate securities		160,598		113		(306	)		160,405
Total	$	326,907	$	166	$	(724	)	$	326,349

As of March 31, 2024, the Company held 113 securities that had been in an unrealized loss position for less than 12 months with an aggregate fair value of $467.4 million. As of December 31, 2023, the Company held 109 securities that had been in an unrealized loss position for less than 12 months with an aggregate fair value of $229.7 million. As of March 31, 2024, the Company held 39 securities that had been in an unrealized loss position for greater than 12 months with an aggregate fair value of $81.5 million. As of December 31, 2023, the Company held 16 securities that had been in an unrealized loss position for greater than 12 months with an aggregate fair value of $36.6 million.

As of March 31, 2024 the Company had 140 securities with a fair value of $427.0 million with a contractual maturity of less than 12 months and 61 securities with a fair value of $224.4 million with a contractual maturity of greater than 12 months. As of December 31, 2023 the Company had 124 securities with a fair value of $264.9 million with a contractual maturity of less than 12 months and 27 securities with a fair value of $61.4 million with a contractual maturity of greater than 12 months.

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 March 31, 2024 and December 31, 2023.