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Marketable Securities (Tables)
 12 Months Ended
Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]  
Held-to-Maturity Securities
The Company's investment in marketable securities as of December 31, 2017 and 2018 are classified as ''held-to-maturity'' and consist of the following:
 
         
Gross
    
Gross
        
         
unrealized
    
unrealized
        
   
Amortized
    
holding
    
holding
    
Aggregate
  
   
cost basis**
    
gains
    
(losses)
    
fair value*
  
   
US$ thousands
  
At December 31, 2018
                        
Held to maturity:
                        
Corporate debt securities and government debt securities
                        
Current
    
1,610
      
-
      
(22
)
    
1,588
  
Non-Current
    
46,052
      
-
      
(778
)
    
45,274
  
                                 
     
47,662
      
-
      
(800
)
    
46,862
  
                                 
At December 31, 2017
                                
Held to maturity:
                                
Corporate debt securities
                                
Current
    
7,798
      
-
      
(49
)
    
7,749
  
Non-Current
    
5,976
      
-
      
(95
)
    
5,881
  
                                 
     
13,774
      
-
      
(144
)
    
13,630
  
 
 
*
Fair value is being determined using quoted market prices in active markets (Level 2).
 
**
Including accrued interest in the amount of US$ 77 thousand and US$ 450 thousand as of December 31, 2017 and 2018 respectively.
The accrued interest is presented as part of other account receivable on the balance sheet.
Schedule of Reconciliation of Marketable Securities
Activity in marketable securities in 2018
  
US$ thousands
  
       
Balance at January 1, 2018
    
13,774
  
         
Purchases of marketable securities
    
41,670
  
Discount on marketable securities, net
    
(32
)
Proceeds from maturity of marketable securities
    
(7,750
)
Balance at December 31, 2018
    
47,662
  
 
Summary of Investment Securities in an Unrealized Loss Position
The following table summarizes the gross unrealized losses on investment securities for which other-than-temporary impairments have not been recognized and the fair value of those securities, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position, at December 31, 2018:
  
 
   
Less than 12 months
    
12 months or more
    
Total
  
Held to maturity:
  
Unrealized Losses
    
Fair value
    
Unrealized Losses
    
Fair value
    
Unrealized Losses
    
Fair value
  
                                     
Corporate debt securities  and government debt  securities
    
(679
)
    
41,024
      
(121
)
    
5,838
      
(800
)
    
46,862