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

June 30, 2021

 
  

Total

  

Level 1

  

Level 2

  

Level 3

 

Money market funds

 $13,124  $13,124  $-  $- 

Certificates of deposit

  108,395   -   108,395   - 

Corporate debt securities

  334,199   -   334,199   - 

U.S. treasuries and government agency bonds

  7,484   -   7,484   - 

Auction-rate securities backed by student-loan notes

  2,582   -   -   2,582 

Mutual funds and money market funds under deferred compensation plan

  29,796   29,796   -   - 

Total

 $495,580  $42,920  $450,078  $2,582 
  

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