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MARKETABLE DEBT SECURITIES
 12 Months Ended
Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]  
MARKETABLE DEBT SECURITIES MARKETABLE DEBT SECURITIES
The following tables summarize our marketable debt securities as of December 31, 2019 and 2018, in thousands:
As of December 31, 2019
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Fair Value
Certificates of deposit$4,303  $—  $(2) $4,301  
Commercial paper39,913  —  —  39,913  
Corporate notes146,016  58  (34) 146,040  
U.S. government-sponsored enterprise securities32,487  3  (2) 32,488  
U.S. treasury securities1,092,293  185  (71) 1,092,407  
Total$1,315,012  $246  $(109) $1,315,149  
As of December 31, 2018
Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Fair Value
Certificates of deposit$8,951  $—  $—  $8,951  
Commercial paper57,197  —  —  57,197  
Corporate notes232,695  —  (285) 232,410  
U.S. government-sponsored enterprise securities39,031  —  (13) 39,018  
U.S. treasury securities546,631  1  (124) 546,508  
Total$884,505  $1  $(422) $884,084  
The fair values of our marketable debt securities by classification in the consolidated balance sheets were as follows, in thousands:
December 31, 2019December 31, 2018
Cash and cash equivalents$340,132  $221,281  
Marketable debt securities975,017  662,803  
Total$1,315,149  $884,084