Marketable Securities

3 Months Ended

Mar. 31, 2021

4. Marketable Securities

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

Description	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
March 31, 2021
U.S. treasury securities	$	288,541	$	16	$	—		$	288,557
Corporate securities		93,557		—		(36	)		93,521
Total	$	382,098	$	16	$	(36	)	$	382,078

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 March 31, 2021, the Company held 32 securities that had been in an unrealized loss position for less than 12 months with an aggregate fair value of $137.7 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 March 31, 2021, the Company had 47 securities with a fair value of $280.4 million with a contractual maturity of less than 12 months and 15 securities with a fair value of $101.6 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 months ending March 31, 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 32 securities in an unrealized loss position as of March 31, 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.