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Note 10 - Fair Value Measurements (Tables)
9 Months Ended
Sep. 30, 2021
Notes Tables  
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block]
  

September 30, 2021

 
  

Total

  

Level 1

  

Level 2

  

Level 3

 

Money market funds

 $13,469  $13,469  $-  $- 

Certificates of deposit

  139,361   -   139,361   - 

Corporate debt securities

  369,094   -   369,094   - 

U.S. treasuries and government agency bonds

  7,492   -   7,492   - 

Auction-rate securities backed by student-loan notes

  2,501   -   -   2,501 

Mutual funds and money market funds under deferred compensation plan

  29,919   29,919   -   - 

Total

 $561,836  $43,388  $515,947  $2,501 
  

December 31, 2020

 
  

Total

  

Level 1

  

Level 2

  

Level 3

 

Money market funds

 $34,335  $34,335  $-  $- 

Corporate debt securities

  249,671   -   249,671   - 

Commercial paper

  2,999   -   2,999   - 

U.S. treasuries and government agency bonds

  7,499   -   7,499   - 

Auction-rate securities backed by student-loan notes

  2,861   -   -   2,861 

Mutual funds and money market funds under deferred compensation plan

  26,924   26,924   -   - 

Total

 $324,289  $61,259  $260,169  $2,861