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Marketable Securities (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Long-term - security bond maturity year [1] Perpetual  
Short-term [2] $ 5,878 $ 4,784
Long-term - security bond [1] 544 832
Municipal bond [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Short-term [2] 4,806 4,117
Corporate bond [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Short-term [2] $ 1,072 $ 667
[1] As of December 31, 2017 the Company held a long-term security bond which is classified as available-for-sale security and presented at fair value. The fair value of the available-for-sale security based on quoted prices in active markets for identical instruments (Level 1 as defined under ASC 820).
[2] The Company invests in highly-rated marketable securities, and its policy limits the amount of credit exposure to any one issuer. The Company's investment policy requires investments to be investment grade, rated single -A or better, with the objective of minimizing the potential risk of principal loss. Fair values were determined for each individual security in the investment portfolio based on quoted prices in active markets.