Marketable Securities

9 Months Ended

Sep. 30, 2021

4. Marketable Securities

The following table summarizes the available-for-sale debt securities held at September 30, 2021 and December 31, 2020 and (in thousands):


Description	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
September 30, 2021
U.S. treasury securities	$	179,723	$	23	$	(5	)	$	179,741
Corporate securities		274,682		5		(169	)		274,518
Total	$	454,405	$	28	$	(174	)	$	454,259


Description	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
December 31, 2020
U.S. treasury securities	$	324,917	$	—	$	(53	)	$	324,864
Corporate securities		102,946		—		(81	)		102,865
Total	$	427,863	$	—	$	(134	)	$	427,729

As of September 30, 2021, the Company held 79 securities that had been in an unrealized loss position for less than 12 months with an aggregate fair value of $286.6 million. As of December 31, 2020, the Company held 65 securities that had been in an unrealized loss position for less than 12 months with an aggregate fair value of $392.2 million.

As of September 30, 2021, the Company had 77 securities with a fair value of $379.6 million with a contractual maturity of less than 12 months and 24 securities with a fair value of $74.7 million with a contractual maturity of greater than 12 months. As of December 31, 2020, the Company had 48 securities with a fair value of $295.6 million with a contractual maturity of less than 12 months and 25 securities with a fair value of $162.5 million with a contractual maturity of greater than 12 months. There were no sales of marketable securities during the three and nine months ending September 30, 2021 and 2020.

The Company evaluates securities for other-than-temporary impairments based on quantitative and qualitative factors, and considered the decline in market value for the 79 securities in an unrealized loss position as of September 30, 2021 to be primarily attributable to the then current economic and market conditions. The Company neither intends to sell these investments nor concludes that it is more-likely-than-not that the Company will have to sell them before recovery of their carrying values. The Company also believes that it will be able to collect both principal and interest amounts due to it at maturity.