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Marketable securities
 9 Months Ended
Sep. 30, 2021
Investments, Debt and Equity Securities [Abstract]  
Marketable securities Marketable securities
The following table summarizes the marketable securities held at September 30, 2021 and December 31, 2020 (in thousands):
Description
Amortized
cost / Cost
Unrealized
gains
Unrealized
losses
Fair
value
September 30, 2021
U.S. government agency securities and treasuries
$312,483 $67 $(58)$312,492 
Corporate bonds
111,616 7 (35)111,588 
Commercial paper
141,089 — — 141,089 
Equity securities
4,305 —(1,205)3,100 
Total
$569,493 $74 $(1,298)$568,269 
December 31, 2020
U.S. government agency securities and treasuries$675,043 $302 $(74)$675,271 
Corporate bonds
197,171 432 (40)197,563 
Commercial paper
77,949 1 — 77,950 
Equity securities
20,017 — (14,364)5,653 
Total
$970,180 $735 $(14,478)$956,437 
No available-for-sale debt securities held as of September 30, 2021 or December 31, 2020 had remaining maturities greater than five years.