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Note 4 - Fair Value Measurements (Tables)
12 Months Ended
Dec. 31, 2021
Notes Tables  
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block]
  

December 31, 2021

 
  

Total

  

Level 1

  

Level 2

  

Level 3

 

Money market funds

 $14,328  $14,328  $-  $- 

Certificates of deposit

  141,478   -   141,478   - 

Corporate debt securities

  386,883   -   386,883   - 

U.S. treasuries and government agency bonds

  7,456   -   7,456   - 

Auction-rate securities backed by student-loan notes

  2,401   -   -   2,401 

Mutual funds and money market funds under deferred compensation plan

  31,786   31,786   -   - 

Total

 $584,332  $46,114  $535,817  $2,401 
  

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