Short-Term Investments

6 Months Ended

Jun. 30, 2021

Investments, Debt and Equity Securities [Abstract]

The Company invests in marketable securities primarily consisting of debt instruments of the U.S. Government, U.S. Government-sponsored enterprises, and financial institutions and corporations with strong credit ratings. The following represents a summary of the estimated fair value of short-term investments as of June 30, 2021 and December 31, 2020 (in thousands):


At June 30, 2021	Maturity (in years)	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss		Estimated Fair Value
Available-for-sale securities:
Commercial paper	Less than 1	$	222,319	$	11	$	(2)	$	222,328
U.S. Government-sponsored enterprise	Less than 2	63,175		18		(19)		63,174
U.S. Treasury securities	Less than 2	104,937		2		(23)		104,916
Corporate debt securities	Less than 2	53,303		6		(13)		53,296
Supranational bonds	Less than 1	3,010		—		—		3,010
Total		$	446,744	$	37	$	(57)	$	446,724


At December 31, 2020	Maturity (in years)	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss		Estimated Fair Value
Available-for-sale securities:
Commercial paper	Less than 1	$	108,892	$	5	$	(1)	$	108,896
U.S. Government-sponsored enterprise	Less than 2	52,330		21		(1)		52,350
U.S. Treasury securities	Less than 2	143,244		12		(2)		143,254
Corporate debt securities	Less than 2	85,788		48		(13)		85,823
Total		$	390,254	$	86	$	(17)	$	390,323

The Company has classified all marketable securities, regardless of maturity, as short-term investments based upon the Company’s ability and intent to use any of those marketable securities to satisfy the Company’s liquidity requirements.

The Company periodically reviews the portfolio of available-for-sale debt securities to determine if any investment is impaired due to changes in credit risk or other potential valuation concerns. Unrealized losses on available-for-sale debt securities at June 30, 2021 were not significant and were due to changes in interest rates, including credit spreads from perceived increased credit risks as a result of the COVID-19 global pandemic. The Company does not intend to sell the available-for-sale debt securities that are in an unrealized loss position, and it is not more likely than not that the Company will be required to sell these debt securities before recovery of their amortized cost bases, which may be at maturity. Based on the credit quality of the available-for-sale debt securities in an unrealized loss position, and the Company’s estimates of future cash flows to be collected from those securities, the Company believes the unrealized losses are not credit losses. Accordingly, the Company did not recognize any impairment losses related to its available-for-sale debt securities at June 30, 2021.