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SHORT-TERM AND LONG-TERM INVESTMENTS (Tables)
 12 Months Ended
Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]  
Summary of Amortized Costs, Gross Unrealized Gains and Losses and Estimated Fair Values of Available-For-Sale Marketable Securities
The following table summarizes amortized costs, gross unrealized gains and losses and estimated fair values of available-for-sale marketable securities as of December 31, 2018 and 2017:

   
Amortized cost
    
Gross unrealized gains
    
Gross unrealized losses
    
Estimated fair value (Level 2 within the fair value hierarchy)
  
   
December 31,
    
December 31,
    
December 31,
    
December 31,
  
   
2018
    
2017
    
2018
    
2017
    
2018
    
2017
    
2018
    
2017
  
                                                 
Corporate debentures
  
$
457,943
    
$
189,836
    
$
190
    
$
9
    
$
1,993
    
$
908
    
$
456,141
    
$
188,937
  
U.S. Treasuries
    
21,944
      
7,007
      
-
      
-
      
226
      
170
      
21,717
      
6,837
  
U.S. Government Agencies
    
10,854
      
998
      
16
      
-
      
1
      
1
      
10,869
      
997
  
                                                                 
   
$
490,741
    
$
197,841
    
$
206
    
$
9
    
$
2,220
    
$
1,079
    
$
488,727
    
$
196,771
  
Scheduled Maturities of Available-for-Sale Marketable Securities
The scheduled maturities of available-for-sale marketable securities as of December 31, 2018 are as follows:
 
   
Amortized
    
Estimated
  
   
cost
    
fair value
  
             
Due within one year
  
$
244,233
    
$
243,729
  
Due after one year through five years
    
246,508
      
244,998
  
                 
   
$
490,741
    
$
488,727
  
Schedule of Unrealized Losses and Fair Values
Investments with continuous unrealized losses for less than 12 months and 12 months or greater and their related fair values as of December 31, 2018 and 2017 are as indicated in the following tables:
 
   
December 31, 2018
  
   
Investments with continuous unrealized losses for less than 12 months
    
Investments with continuous unrealized losses for 12 months or greater
    
Total Investments with continuous unrealized losses
  
   
Fair
value
    
Unrealized losses
    
Fair
value
    
Unrealized losses
    
Fair
value
    
Unrealized losses
  
                                     
Corporate debentures
  
$
231,845
    
$
(754
)
  
$
113,870
    
$
(1,239
)
  
$
345,715
    
$
(1,993
)
U.S. treasuries
    
14,926
      
(12
)
    
6,791
      
(214
)
    
21,717
      
(226
)
U.S. Government agencies
    
7,932
      
(1
)
    
-
      
-
      
7,932
      
(1
)
                                                 
   
$
254,703
    
$
(767
)
  
$
120,661
    
$
(1,453
)
  
$
375,364
    
$
(2,220
)

   
December 31, 2017
  
   
Investments with continuous unrealized losses for less than 12 months
    
Investments with continuous unrealized losses for 12 months or greater
    
Total Investments with continuous unrealized losses
  
   
Fair
value
    
Unrealized losses
    
Fair
value
    
Unrealized losses
    
Fair
value
    
Unrealized losses
  
                                     
Corporate debentures
  
$
135,252
    
$
(711
)
  
$
49,076
    
$
(198
)
  
$
184,328
    
$
(909
)
U.S. treasuries
    
-
      
-
      
6,837
      
(170
)
    
6,837
      
(170
)
U.S. Government agencies
    
997
      
-
      
-
      
-
      
997
      
-
  
                                                 
   
$
136,249
    
$
(711
)
  
$
55,913
    
$
(368
)
  
$
192,162
    
$
(1,079
)