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Marketable Securities
 9 Months Ended
Sep. 30, 2020
Investments, Debt and Equity Securities [Abstract]  
Marketable securities Marketable securities
The following table summarizes the marketable securities held at September 30, 2020 and December 31, 2019 (in thousands):
Description
Amortized
cost / Cost
Unrealized
gains
Unrealized
losses
Fair
value
September 30, 2020
U.S. government agency securities and treasuries
$778,742 $539 $(223)$779,058 
Corporate bonds
237,884 1,079 (58)238,905 
Commercial paper
91,939 2 — 91,941 
Equity securities
20,017 —(16,215)3,802 
Total
$1,128,582 $1,620 $(16,496)$1,113,706 
December 31, 2019
U.S. government agency securities and treasuries$633,970 $2,014 $(48)$635,936 
Certificates of deposit
960 — — 960 
Corporate bonds
185,827 824 (43)186,608 
Commercial paper
74,378 — — 74,378 
Equity securities
20,017 — (7,147)12,870 
Total
$915,152 $2,838 $(7,238)$910,752 
No available-for-sale debt securities held as of September 30, 2020 or December 31, 2019 had remaining maturities greater than five years.