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Marketable Securities (Tables) | 12 Months Ended
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Dec. 31, 2017
Marketable Securities [Abstract] |
Schedule of marketable securities |
|  |  |  |  | December 31, |
|  |  | Maturity year |  | 2 0 1 7 |  |  | 2 0 1 6 |
|  |  |  |  | U.S. dollars in thousands |
| Short-term (a): |  |  |  |  |  |  |  |
| Municipal bond |  |  |  | $ | 4,806 |  |  | $ | 4,117 |
| Corporate bond |  |  |  |  | 1,072 |  |  |  | 667 |
|  |  |  |  | $ | 5,878 |  |  | $ | 4,784 |
| Long-term - security bond (b) |  | Perpetual |  | $ | 544 |  |  | $ | 832 |
| a. | 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.
| b. | 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).

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- Definition Tabular disclosure of the reconciliation of available-for-sale securities from cost basis to fair value. + References No definition available. + Details
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