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Note 10 - Fair Value Measurements (Tables)
3 Months Ended
Mar. 31, 2021
Notes Tables  
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block]
  

March 31, 2021

 
  

Total

  

Level 1

  

Level 2

  

Level 3

 

Money market funds

 $20,624  $20,624  $-  $- 

Certificates of deposit

  106,763   -   106,763   - 

Corporate debt securities

  306,201   -   306,201   - 

U.S. treasuries and government agency bonds

  7,491   -   7,491   - 

Auction-rate securities backed by student-loan notes

  2,820   -   -   2,820 

Mutual funds and money market funds under deferred compensation plan

  28,252   28,252   -   - 

Total

 $472,151  $48,876  $420,455  $2,820 
  

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