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Note 10 - Fair Value Measurements (Tables)
3 Months Ended
Mar. 31, 2023
Notes Tables  
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block]
  

March 31, 2023

 
  

Total

  

Level 1

  

Level 2

  

Level 3

 

Money market funds

 $41,226  $41,226  $-  $- 

Certificates of deposit

  131,042   -   131,042   - 

Corporate debt securities

  244,155   -   244,155   - 

Commercial paper

  18,163   -   18,163    

U.S. treasuries and government agency bonds

  35,238   -   35,238   - 

Auction-rate securities backed by student-loan notes

  1,516   -   -   1,516 

Mutual funds and money market funds under deferred compensation plan

  47,114   47,114   -   - 

Total

 $518,454  $88,340  $428,598  $1,516 
  

December 31, 2022

 
  

Total

  

Level 1

  

Level 2

  

Level 3

 

Money market funds

 $15,462  $15,462  $-  $- 

Certificates of deposit

  130,467   -   130,467   - 

Corporate debt securities

  292,586   -   292,586   - 

Commercial paper

  17,928   -   17,928   - 

U.S. treasuries and government agency bonds

  8,285   -   8,285   - 

Auction-rate securities backed by student-loan notes

  1,711   -   -   1,711 

Mutual funds and money market funds under deferred compensation plan

  43,933   43,933   -   - 

Total

 $510,372  $59,395  $449,266  $1,711