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Note 10 - Fair Value Measurements (Tables)
9 Months Ended
Sep. 30, 2020
Notes Tables  
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block]
  

September 30, 2020

 
  

Total

  

Level 1

  

Level 2

  

Level 3

 

Money market funds

 $41,195  $41,195  $-  $- 

Corporate debt securities

  349,342   -   349,342   - 

Commercial paper

  7,985   -   7,985   - 

U.S. treasuries and government agency bonds

  14,749   -   14,749   - 

Auction-rate securities backed by student-loan notes

  2,916   -   -   2,916 

Mutual funds and money market funds under deferred compensation plan

  23,562   23,562   -   - 

Total

 $439,749  $64,757  $372,076  $2,916 
  

December 31, 2019

 
  

Total

  

Level 1

  

Level 2

  

Level 3

 

Money market funds

 $28,100  $28,100  $-  $- 

Corporate debt securities

  260,950   -   260,950   - 

Commercial paper

  1,994   -   1,994   - 

U.S. treasuries and government agency bonds

  19,493   -   19,493   - 

Auction-rate securities backed by student-loan notes

  3,138   -   -   3,138 

Mutual funds and money market funds under deferred compensation plan

  21,975   21,975   -   - 

Total

 $335,650  $50,075  $282,437  $3,138 
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block]

Balance at January 1, 2020

 $3,138 

Change in unrealized gain included in other comprehensive income

  28 

Sale and settlement at par

  (250)

Balance at September 30, 2020

 $2,916 
Fair Value Measurement Inputs and Valuation Techniques [Table Text Block]
  

September 30, 2020 (1)

 

December 31, 2019

 

Time-to-liquidity (in years)

 2-3(2.5) 2-3 

Discount rate

 3.3%-7.6%(5.4%) 4.0%-8.3%