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MARKETABLE DEBT SECURITIES
 12 Months Ended
Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]  
MARKETABLE DEBT SECURITIES MARKETABLE DEBT SECURITIES
The following tables summarize our marketable debt securities:
As of December 31, 2020
(In thousands)Amortized
Cost		Gross
Unrealized
Gains		Gross
Unrealized
Losses		Fair Value
U.S. government-sponsored enterprise securities$245,113 $135 $(34)$245,214 
U.S. treasury securities1,107,721 328 (81)1,107,968 
Total$1,352,834 $463 $(115)$1,353,182 

As of December 31, 2019
(In thousands)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 
The fair values of our marketable debt securities by classification in the consolidated balance sheets were as follows:
(In thousands)December 31, 2020December 31, 2019
Cash and cash equivalents$20,000 $340,132 
Marketable debt securities1,333,182 975,017 
Total$1,353,182 $1,315,149